Mobile object tracking in wireless sensor networks (WSNs) has gained much attention during recent years due to the special characteristics of these networks. Considering the wide application of tracking in WSNs, various problems of data aggregation, routing, scheduling and energy conservation have been revisited and new solutions have been proposed recently. Due to the importance of this topic and the availability of a significant body of literature, a detailed survey becomes necessary and useful at this stage. In this paper we present a survey of the state-of-the-art mobile object tracking techniques in WSNs. We give a definition for the general problem of object tracking and introduce challenges in designing an efficient object tracking sensor network (OTSN). We highlight the advantages and performance issues of the existing tracking methods and make a brief comparison based on the design parameters proposed. Finally, we will present open problems and the future works conceivable in this broad field of research.