Wireless Sensor Networks (WSNs) have become an active research area for the researchers. Although sensor nodes have low processor, low memory and limited energy, they have capabilities with random located, self organizing, collective work, and local computation. WSNs consist of these nodes. WSNs are feasible in military, healthcare, environmental, home automotion and commercial applications. Various Medium Access Control (MAC) protocols with different objectives were proposed for WSNs. In this paper, we first outline the sensor network properties that are crucial for the design of MAC layer protocols. Then, we describe several MAC protocols proposed for sensor networks emphasizing their strengths and weaknesses. Also, we have presented security problems and solutions on MAC protocol. Finally, we point out open research issues of MAC layer design. It is considered that this study will help to MAC protocol designers.