A Wireless Sensor Network is a network of randomly deployed sensor nodes that cooperatively monitor the physical and environmental conditions. Such networks face some critical issues like limited battery power, limited network lifetime and deployment in drastic environmental conditions. The performance of Wireless Sensor Network largely depends on the routing protocols. The lifetime of the sensor network primarily depends on the residual battery of each sensor node. Many routing protocols have been proposed to optimize the energy efficiency of the Sensor Networks. We have carried out an extensive survey on WSN routing protocols. Clustering routing protocols out perform all other routing protocols in terms of extending the network lifetime. This paper mainly focuses on an in-depth survey of hierarchical or clustering routing protocols like LEACH and PEGASIS and their descendants.