diff --git a/src/module.c b/src/module.c
index 347ddabf..0e7fedc1 100644
--- a/src/module.c
+++ b/src/module.c
@@ -65,6 +65,10 @@ struct RedisModuleKey {
     robj *value;    /* Value object, or NULL if the key was not found. */
     void *iter;     /* Iterator. */
     int mode;       /* Opening mode. */
+    /* Zset iterator. */
+    RedisModuleZsetRange *zr;   /* Zset iterator range passed by user. */
+    void *zcurrent; /* Zset iterator current node. */
+    int zer; /* Zset iterator end reached flag (true if end was reached). */
 };
 typedef struct RedisModuleKey RedisModuleKey;
 
@@ -115,6 +119,7 @@ void RM_CloseKey(RedisModuleKey *key);
 void RM_AutoMemoryCollect(RedisModuleCtx *ctx);
 robj **moduleCreateArgvFromUserFormat(const char *cmdname, const char *fmt, int *argcp, int *flags, va_list ap);
 void moduleReplicateMultiIfNeeded(RedisModuleCtx *ctx);
+void RM_ZsetRangeStop(RedisModuleKey *key);
 
 /* --------------------------------------------------------------------------
  * Helpers for modules API implementation
@@ -380,7 +385,7 @@ const char *RM_StringPtrLen(RedisModuleString *str, size_t *len) {
     return str->ptr;
 }
 
-/* Turn the string into a long long, storing it at *ll if not NULL.
+/* Turn the string into a long long, storing it at *ll.
  * Returns REDISMODULE_OK on success. If the string can't be parsed
  * as a valid, strict long long (no spaces before/after), REDISMODULE_ERR
  * is returned. */
@@ -389,6 +394,14 @@ int RM_StringToLongLong(RedisModuleString *str, long long *ll) {
                                                      REDISMODULE_ERR;
 }
 
+/* Turn the string into a double, storing it at *d.
+ * Returns REDISMODULE_OK on success or REDISMODULE_ERR if the string is
+ * not a valid string representation of a double value. */
+int RM_StringToDouble(RedisModuleString *str, double *d) {
+    int retval = getDoubleFromObject(str,d);
+    return (retval == C_OK) ? REDISMODULE_OK : REDISMODULE_ERR;
+}
+
 /* --------------------------------------------------------------------------
  * Reply APIs
  *
@@ -480,6 +493,12 @@ int RM_ReplyWithCallReply(RedisModuleCtx *ctx, RedisModuleCallReply *reply) {
     return REDISMODULE_OK;
 }
 
+/* Send a string reply obtained converting the double 'd' into a string. */
+int RM_ReplyWithDouble(RedisModuleCtx *ctx, double d) {
+    addReplyDouble(ctx->client,d);
+    return REDISMODULE_OK;
+}
+
 /* --------------------------------------------------------------------------
  * Commands replication API
  * -------------------------------------------------------------------------- */
@@ -592,6 +611,7 @@ void *RM_OpenKey(RedisModuleCtx *ctx, robj *keyname, int mode) {
     kp->value = value;
     kp->iter = NULL;
     kp->mode = mode;
+    RM_ZsetRangeStop(kp);
     RM_AutoMemoryAdd(ctx,REDISMODULE_AM_KEY,kp);
     return (void*)kp;
 }
@@ -971,6 +991,119 @@ int RM_ZsetScore(RedisModuleKey *key, RedisModuleString *ele, double *score) {
     return REDISMODULE_OK;
 }
 
+/* --------------------------------------------------------------------------
+ * Key API for Sorted Set iterator
+ * -------------------------------------------------------------------------- */
+
+/* Stop a sorted set iteration. */
+void RM_ZsetRangeStop(RedisModuleKey *key) {
+    /* Setup sensible values so that misused iteration API calls when an
+     * iterator is not active will result into something more sensible
+     * than crashing. */
+    key->zr = NULL;
+    key->zcurrent = NULL;
+    key->zer = 1;
+}
+
+/* Return the "End of range" flag value to signal the end of the iteration. */
+int RM_ZsetRangeEndReached(RedisModuleKey *key) {
+    return key->zer;
+}
+
+/* Setup a sorted set iterator seeking the first element in the specified
+ * range. Returns REDISMODULE_OK if the iterator was correctly initialized
+ * otherwise REDISMODULE_ERR is returned in the following conditions:
+ *
+ * 1. The value stored at key is not a sorted set or the key is empty.
+ * 2. The iterator type is unrecognized. */
+int RM_ZsetFirstInRange(RedisModuleKey *key, RedisModuleZsetRange *zr) {
+    if (!key->value || key->value->type != OBJ_ZSET) return REDISMODULE_ERR;
+    key->zr = zr;
+    key->zcurrent = NULL;
+    key->zer = 0;
+
+    if (zr->type == REDISMODULE_ZSET_RANGE_SCORE) {
+        /* Setup the range structure used by the sorted set core implementation
+         * in order to seek at the specified element. */
+        zrangespec zrs;
+        zrs.min = zr->score_start;
+        zrs.max = zr->score_end;
+        zrs.minex = (zr->flags & REDISMODULE_ZSET_RANGE_START_EX) != 0;
+        zrs.maxex = (zr->flags & REDISMODULE_ZSET_RANGE_END_EX) != 0;
+
+        if (key->value->encoding == OBJ_ENCODING_ZIPLIST) {
+            key->zcurrent = zzlFirstInRange(key->value->ptr,&zrs);
+        } else if (key->value->encoding == OBJ_ENCODING_SKIPLIST) {
+            zset *zs = key->value->ptr;
+            zskiplist *zsl = zs->zsl;
+            key->zcurrent = zslFirstInRange(zsl,&zrs);
+        } else {
+            serverPanic("Unsupported zset encoding");
+        }
+        if (key->zcurrent == NULL) key->zer = 1;
+        return REDISMODULE_OK;
+    } else {
+        return REDISMODULE_ERR;
+    }
+}
+
+/* Return the current sorted set element of an active sorted set iterator
+ * or NULL if the range specified in the iterator does not include any
+ * element. */
+RedisModuleString *RM_ZsetRangeCurrentElement(RedisModuleKey *key, double *score) {
+    if (key->zcurrent == NULL) return NULL;
+    if (key->value->encoding == OBJ_ENCODING_ZIPLIST) {
+        unsigned char *eptr, *sptr;
+        eptr = key->zcurrent;
+        sds ele = ziplistGetObject(eptr);
+        if (score) {
+            sptr = ziplistNext(key->value->ptr,eptr);
+            *score = zzlGetScore(sptr);
+        }
+        return createObject(OBJ_STRING,ele);
+    } else if (key->value->encoding == OBJ_ENCODING_SKIPLIST) {
+        zskiplistNode *ln = key->zcurrent;
+        if (score) *score = ln->score;
+        return createStringObject(ln->ele,sdslen(ln->ele));
+    } else {
+        serverPanic("Unsupported zset encoding");
+    }
+}
+
+/* Go to the next element of the sorted set iterator. Returns 1 if there was
+ * a next element, 0 if we are already at the latest element or the range
+ * does not include any item at all. */
+int RM_ZsetRangeNext(RedisModuleKey *key) {
+    if (!key->zr || !key->zcurrent) return 0; /* No active iterator. */
+    if (key->value->encoding == OBJ_ENCODING_ZIPLIST) {
+        unsigned char *zl = key->value->ptr;
+        unsigned char *eptr = key->zcurrent;
+        unsigned char *next;
+        next = ziplistNext(zl,eptr); /* Skip element. */
+        if (next) next = ziplistNext(zl,next); /* Skip score. */
+        if (next == NULL) {
+            key->zer = 1;
+            return 0;
+        } else {
+            /* TODO: check if we are in range. */
+            key->zcurrent = next;
+            return 1;
+        }
+    } else if (key->value->encoding == OBJ_ENCODING_SKIPLIST) {
+        zskiplistNode *ln = key->zcurrent, *next = ln->level[0].forward;
+        if (next == NULL) {
+            key->zer = 1;
+            return 0;
+        } else {
+            /* TODO: check if we are in range. */
+            key->zcurrent = next;
+            return 1;
+        }
+    } else {
+        serverPanic("Unsupported zset encoding");
+    }
+}
+
 /* --------------------------------------------------------------------------
  * Redis <-> Modules generic Call() API
  * -------------------------------------------------------------------------- */
@@ -1385,6 +1518,7 @@ void moduleRegisterCoreAPI(void) {
     REGISTER_API(ReplyWithStringBuffer);
     REGISTER_API(ReplyWithNull);
     REGISTER_API(ReplyWithCallReply);
+    REGISTER_API(ReplyWithDouble);
     REGISTER_API(GetSelectedDb);
     REGISTER_API(SelectDb);
     REGISTER_API(OpenKey);
@@ -1394,6 +1528,7 @@ void moduleRegisterCoreAPI(void) {
     REGISTER_API(ListPush);
     REGISTER_API(ListPop);
     REGISTER_API(StringToLongLong);
+    REGISTER_API(StringToDouble);
     REGISTER_API(Call);
     REGISTER_API(CallReplyProto);
     REGISTER_API(FreeCallReply);
@@ -1420,6 +1555,11 @@ void moduleRegisterCoreAPI(void) {
     REGISTER_API(ZsetIncrby);
     REGISTER_API(ZsetScore);
     REGISTER_API(ZsetRem);
+    REGISTER_API(ZsetRangeStop);
+    REGISTER_API(ZsetFirstInRange);
+    REGISTER_API(ZsetRangeCurrentElement);
+    REGISTER_API(ZsetRangeNext);
+    REGISTER_API(ZsetRangeEndReached);
 }
 
 /* Global initialization at Redis startup. */
diff --git a/src/modules/helloworld.c b/src/modules/helloworld.c
index 7480415e..5f436609 100644
--- a/src/modules/helloworld.c
+++ b/src/modules/helloworld.c
@@ -326,6 +326,39 @@ int HelloMoreExpire_RedisCommand(RedisModuleCtx *ctx, RedisModuleString **argv,
     return RedisModule_ReplyWithSimpleString(ctx,"OK");
 }
 
+/* HELLO.ZSUMRANGE key startscore endscore
+ * Return the sum of all the scores elements between startscore and endscore. */
+int HelloZsumRange_RedisCommand(RedisModuleCtx *ctx, RedisModuleString **argv, int argc) {
+    RedisModuleZsetRange zrange = REDISMODULE_ZSET_RANGE_INIT;
+    zrange.type = REDISMODULE_ZSET_RANGE_SCORE;
+    if (RedisModule_StringToDouble(argv[2],&zrange.score_start) != REDISMODULE_OK ||
+        RedisModule_StringToDouble(argv[3],&zrange.score_end) != REDISMODULE_OK)
+    {
+        return RedisModule_ReplyWithError(ctx,"ERR invalid range");
+    }
+    zrange.flags = 0;
+
+    RedisModuleKey *key = RedisModule_OpenKey(ctx,argv[1],
+        REDISMODULE_READ|REDISMODULE_WRITE);
+    if (RedisModule_KeyType(key) != REDISMODULE_KEYTYPE_ZSET) {
+        return RedisModule_ReplyWithError(ctx,REDISMODULE_ERRORMSG_WRONGTYPE);
+    }
+
+    RedisModule_ZsetFirstInRange(key,&zrange);
+    double scoresum = 0;
+    while(!RedisModule_ZsetRangeEndReached(key)) {
+        double score;
+        RedisModuleString *ele = RedisModule_ZsetRangeCurrentElement(key,&score);
+        RedisModule_FreeString(ctx,ele);
+        scoresum += score;
+        RedisModule_ZsetRangeNext(key);
+    }
+    RedisModule_ZsetRangeStop(key);
+    RedisModule_CloseKey(key);
+    RedisModule_ReplyWithDouble(ctx,scoresum);
+    return REDISMODULE_OK;
+}
+
 /* This function must be present on each Redis module. It is used in order to
  * register the commands into the Redis server. */
 int RedisModule_OnLoad(RedisModuleCtx *ctx) {
@@ -380,5 +413,9 @@ int RedisModule_OnLoad(RedisModuleCtx *ctx) {
         HelloMoreExpire_RedisCommand) == REDISMODULE_ERR)
         return REDISMODULE_ERR;
 
+    if (RedisModule_CreateCommand(ctx,"hello.zsumrange",
+        HelloZsumRange_RedisCommand) == REDISMODULE_ERR)
+        return REDISMODULE_ERR;
+
     return REDISMODULE_OK;
 }
diff --git a/src/redismodule.h b/src/redismodule.h
index fcc53e50..42058b99 100644
--- a/src/redismodule.h
+++ b/src/redismodule.h
@@ -2,6 +2,7 @@
 #define REDISMODULE_H
 
 #include <sys/types.h>
+#include <stdint.h>
 #include <stdio.h>
 
 /* ---------------- Defines common between core and modules --------------- */
@@ -49,6 +50,31 @@
 /* Error messages. */
 #define REDISMODULE_ERRORMSG_WRONGTYPE "WRONGTYPE Operation against a key holding the wrong kind of value"
 
+/* Sorted set range structure. */
+typedef struct RedisModuleZsetRange {
+    uint32_t type;
+    uint32_t flags;
+    double score_start;
+    double score_end;
+    char *lex_start;
+    char *lex_end;
+    uint32_t lex_start_len;
+    uint32_t lex_end_len;
+    uint32_t pos_start;
+    uint32_t pos_end;
+} RedisModuleZsetRange;
+
+#define REDISMODULE_POSITIVE_INFINITE (1.0/0.0)
+#define REDISMODULE_NEGATIVE_INFINITE (-1.0/0.0)
+
+#define REDISMODULE_ZSET_RANGE_INIT {0,0,REDISMODULE_NEGATIVE_INFINITE,REDISMODULE_POSITIVE_INFINITE,"-","+",1,1,0,-1}
+#define REDISMODULE_ZSET_RANGE_LEX 1
+#define REDISMODULE_ZSET_RANGE_SCORE 2
+#define REDISMODULE_ZSET_RANGE_POS 3
+
+#define REDISMODULE_ZSET_RANGE_START_EX (1<<0)
+#define REDISMODULE_ZSET_RANGE_END_EX (1<<1)
+
 /* ------------------------- End of common defines ------------------------ */
 
 #ifndef REDISMODULE_CORE
@@ -98,8 +124,10 @@ int REDISMODULE_API_FUNC(RedisModule_ReplyWithArray)(RedisModuleCtx *ctx, int le
 int REDISMODULE_API_FUNC(RedisModule_ReplyWithStringBuffer)(RedisModuleCtx *ctx, const char *buf, size_t len);
 int REDISMODULE_API_FUNC(RedisModule_ReplyWithString)(RedisModuleCtx *ctx, RedisModuleString *str);
 int REDISMODULE_API_FUNC(RedisModule_ReplyWithNull)(RedisModuleCtx *ctx);
+int REDISMODULE_API_FUNC(RedisModule_ReplyWithDouble)(RedisModuleCtx *ctx, double d);
 int REDISMODULE_API_FUNC(RedisModule_ReplyWithCallReply)(RedisModuleCtx *ctx, RedisModuleCallReply *reply);
 int REDISMODULE_API_FUNC(RedisModule_StringToLongLong)(RedisModuleString *str, long long *ll);
+int REDISMODULE_API_FUNC(RedisModule_StringToDouble)(RedisModuleString *str, double *d);
 void REDISMODULE_API_FUNC(RedisModule_AutoMemory)(RedisModuleCtx *ctx);
 int REDISMODULE_API_FUNC(RedisModule_Replicate)(RedisModuleCtx *ctx, const char *cmdname, const char *fmt, ...);
 int REDISMODULE_API_FUNC(RedisModule_ReplicateVerbatim)(RedisModuleCtx *ctx);
@@ -115,6 +143,11 @@ int REDISMODULE_API_FUNC(RedisModule_ZsetAdd)(RedisModuleKey *key, double score,
 int REDISMODULE_API_FUNC(RedisModule_ZsetIncrby)(RedisModuleKey *key, double score, RedisModuleString *ele, int *flagsptr, double *newscore);
 int REDISMODULE_API_FUNC(RedisModule_ZsetScore)(RedisModuleKey *key, RedisModuleString *ele, double *score);
 int REDISMODULE_API_FUNC(RedisModule_ZsetRem)(RedisModuleKey *key, RedisModuleString *ele, int *deleted);
+void REDISMODULE_API_FUNC(RedisModule_ZsetRangeStop)(RedisModuleKey *key);
+int REDISMODULE_API_FUNC(RedisModule_ZsetFirstInRange)(RedisModuleKey *key, RedisModuleZsetRange *zr);
+RedisModuleString *REDISMODULE_API_FUNC(RedisModule_ZsetRangeCurrentElement)(RedisModuleKey *key, double *score);
+int REDISMODULE_API_FUNC(RedisModule_ZsetRangeNext)(RedisModuleKey *key);
+int REDISMODULE_API_FUNC(RedisModule_ZsetRangeEndReached)(RedisModuleKey *key);
 
 /* This is included inline inside each Redis module. */
 static int RedisModule_Init(RedisModuleCtx *ctx, const char *name, int ver, int apiver) {
@@ -131,6 +164,7 @@ static int RedisModule_Init(RedisModuleCtx *ctx, const char *name, int ver, int
     REDISMODULE_GET_API(ReplyWithString);
     REDISMODULE_GET_API(ReplyWithNull);
     REDISMODULE_GET_API(ReplyWithCallReply);
+    REDISMODULE_GET_API(ReplyWithDouble);
     REDISMODULE_GET_API(GetSelectedDb);
     REDISMODULE_GET_API(SelectDb);
     REDISMODULE_GET_API(OpenKey);
@@ -140,6 +174,7 @@ static int RedisModule_Init(RedisModuleCtx *ctx, const char *name, int ver, int
     REDISMODULE_GET_API(ListPush);
     REDISMODULE_GET_API(ListPop);
     REDISMODULE_GET_API(StringToLongLong);
+    REDISMODULE_GET_API(StringToDouble);
     REDISMODULE_GET_API(Call);
     REDISMODULE_GET_API(CallReplyProto);
     REDISMODULE_GET_API(FreeCallReply);
@@ -166,6 +201,11 @@ static int RedisModule_Init(RedisModuleCtx *ctx, const char *name, int ver, int
     REDISMODULE_GET_API(ZsetIncrby);
     REDISMODULE_GET_API(ZsetScore);
     REDISMODULE_GET_API(ZsetRem);
+    REDISMODULE_GET_API(ZsetRangeStop);
+    REDISMODULE_GET_API(ZsetFirstInRange);
+    REDISMODULE_GET_API(ZsetRangeCurrentElement);
+    REDISMODULE_GET_API(ZsetRangeNext);
+    REDISMODULE_GET_API(ZsetRangeEndReached);
 
     RedisModule_SetModuleAttribs(ctx,name,ver,apiver);
     return REDISMODULE_OK;
diff --git a/src/server.h b/src/server.h
index aa42b100..9cd8661f 100644
--- a/src/server.h
+++ b/src/server.h
@@ -1235,6 +1235,7 @@ int getLongFromObjectOrReply(client *c, robj *o, long *target, const char *msg);
 int checkType(client *c, robj *o, int type);
 int getLongLongFromObjectOrReply(client *c, robj *o, long long *target, const char *msg);
 int getDoubleFromObjectOrReply(client *c, robj *o, double *target, const char *msg);
+int getDoubleFromObject(robj *o, double *target);
 int getLongLongFromObject(robj *o, long long *target);
 int getLongDoubleFromObject(robj *o, long double *target);
 int getLongDoubleFromObjectOrReply(client *c, robj *o, long double *target, const char *msg);
@@ -1333,6 +1334,7 @@ zskiplistNode *zslLastInRange(zskiplist *zsl, zrangespec *range);
 double zzlGetScore(unsigned char *sptr);
 void zzlNext(unsigned char *zl, unsigned char **eptr, unsigned char **sptr);
 void zzlPrev(unsigned char *zl, unsigned char **eptr, unsigned char **sptr);
+unsigned char *zzlFirstInRange(unsigned char *zl, zrangespec *range);
 unsigned int zsetLength(robj *zobj);
 void zsetConvert(robj *zobj, int encoding);
 void zsetConvertToZiplistIfNeeded(robj *zobj, size_t maxelelen);
@@ -1341,6 +1343,7 @@ unsigned long zslGetRank(zskiplist *zsl, double score, sds o);
 int zsetAdd(robj *zobj, double score, sds ele, int *flags, double *newscore);
 long zsetRank(robj *zobj, sds ele, int reverse);
 int zsetDel(robj *zobj, sds ele);
+sds ziplistGetObject(unsigned char *sptr);
 
 /* Core functions */
 int freeMemoryIfNeeded(void);