Wireless Sensor Networks are energy constrained networks that are deployed to sense an environment. The battery of the nodes in these networks depletes with the network operational time and eventually the nodes die out of battery. It becomes very important to minimize the energy consumption in these networks as it is nearly impossible to replace the battery of the dead nodes. Energy consumption can be reduced by employing clustering techniques. The crux of any clustering approach is the cluster head selection mechanism. This paper briefly surveys the cluster head selection techniques used in both homogeneous and heterogeneous WSNs.