In wireless sensor networks, the aim of storage protocols is to efficiently replicate data across nodes and to improvedata collection and querying by sinks. Among them, in-network storage protocols replicate data in a set of nodes thatdepends on some characteristics such as network topology and geographic location. Researchers have proposed varioustechniques to implement in-network storage. In this article, we summarize and highlight the key ideas of existing proto-cols which are further classified into three categories (reactive, unstructured proactive, and structured proactive) basedon topology, load balancing, transmission strategy, and reliability. Benefits and drawbacks of each protocol are studiedand compared with different requirements. Finally, future research directions are provided for efficient in-network stor-age in wireless sensor network.