The recent development of Wireless Sensor Networks (WSN) is created a hot research area that attracts many researchers. WSNs have a wide range of application areas including military, chemical, environmental and industry fields. These networks can be used for applications such as surveillance, target detection and tracking. One of the biggest problems of WSN is that sensor nodes have strictly limited resources. The main limited resource is the battery life. In the literature, many studies have been conducted to increase the lifetime of sensor nodes (i.e., extend the battery life). Due to availability of mobile nodes and mission critical applications of WSNs, mobility and security are other important issues in WSNs. Problem of shared medium access is directly related to these issues and hence there are several studies on medium access protocols in the literature. In this study, medium access protocols of WSNs are examined and evaluated with respect to energy efficiency, mobility and security.