Increased demand of Wireless Sensor Networks (WSN) in various applications has made it a hot research area. Several challenges imposed which include energy conservation, scalability, limited network resources etc. with energy conservation being the most important. Clustering improves the energy efficiency by making high power nodes as cluster heads (CHs) which reduces the chance of energy depletion of nodes. Scalability, fault tolerance, data aggregation, energy efficiency are some of the main objectives of clustering. This paper discusses various challenges associated with clustering and different methods or techniques developed to overcome these challenges. Various clustering approaches have been summarized and few prominent Quality of service (QoS) based clustering routing protocols for WSN have been identified. Comparison of these approaches and protocols is discussed based on some parameters.