Abstract: A Wireless Sensor Network (WSN) is a special kind of ad hoc network with distributed sensing and processing capability. Routing protocols for wireless sensor networks are responsible for maintaining the routes in the network and have to ensure reliable multi-hop communication. Studies from 2006 until early 2015 distinguished three categories of routing protocols i.e., network structure, routing paradigms and initiator of communication. The routing protocols belonging to the first category further classified as flat, hierarchical and location based routing protocols, the routing paradigms classified as single path and multipath and the initiator of communication classified into source initiator and destination initiator routing protocols. The operations of representative members from each category are described and the comparative analyses of the performance of each routing protocol are presented. A literature has been extensively studied to highlight the properties and limitations of existing routing protocols and future research issues, which helps to improve the routing protocols for another domain.