Wireless sensor network (WSN) has emerged as a useful supplement to the modern wireless communication networks. Optimal selection of paths for data transfer results in saving of energy consumption resulting in increase of network lifetime of Wireless Sensor Networks. Many routing, power management, and data dissemination protocols have been specifically designed for WSNs where energy awareness is an essential design issue. Routing protocols in WSNs might differ depending on the application and network architecture as there is still no consensus on a fixed communication stack for WSN. Newer Routing protocols are required to cater to the need of ubiquitous and pervasive computing. In this paper, WSN Routing Protocols has been classified in four ways i.e., routing paths establishment, network structure, protocol operation and initiator of communications. Further, routing protocols have been categorized on the basis of their homogeneity and heterogeneity of sensor nodes followed by the criteria of clustered and non – clustered among both. Data aggregation, support for query and scalability of the network of these routing protocols have also been.