Energy efficiency is the major concern during the design of BSNs. Normally the sensors are powered by batteries and the batteries cannot be replaced if they are inside the human body. The demand for higher resolution and higher sampling frequency of the sensing data makes sensor batteries a big challenge. On the other hand, the path-loss of the body tissue is much larger than through the air, therefore the sensors need larger transmission power to send data. It is well known that the medium access control (MAC) protocol plays an important role in energy management. It can effectively reduce the energy consumption of sensor nodes and prolong the network lifetime. One can focus on the MAC protocol design of BSNs to improve the energy efficiency. The BSN domain can benefit considerably from the appropriate modification and application of modern advances in protocols, techniques and methodologies which have recently been proposed for WSNs pursuing performance improvement in energy efficient routing, QoS, congestion control, congestion avoidance, connectivity and coverage as described in [6–11]. Many energy efficient MAC protocols have already been proposed for WSNs [12–15], however, BSNs have their own attributes which make them different from WSNs [16]. Firstly, WSNs cover a monitoring environment of meters or kilometers, while the scale of BSNs is only as large as the human body (centimeters). The feature makes the coordinator easily reach every sensor node directly. Secondly, WSNs have greater numbers of nodes, ranging from tens to thousands. BSNs have normally less than 10 sensor nodes. Thirdly, BSNs have more energy and processing resource constraints than WSNs. Compared with general BSNs, swallowable BSNs have two main specific features. One is that all the communications are in-vivo. The path-loss through body tissue is different from through the air. The other one is that some of the sensor nodes are mobile. The path-loss between sensor nodes varies. The routing may change. It is necessary to consider the above issue and define the specific MAC protocols for specific BSNs.