In wireless sensor network, Routing is the process by which the data gathered by sensors is relayed towards the end user (usually termed as sink).A lot of routing protocols have been developed since now and these protocols differ according to network structure and field of application. In this paper, a survey of the routing protocols developed so far in field of WSN is presented. Broadly, routing protocols are classified depending on network structure and protocol operation. The advantages and performance issues of hierarchical protocols will also be highlighted.