Wireless sensor networks (WSNs) have become an active research area for the researchers. The sensor nodes are generally unattended after their deployment in hazardous, hostile or remote areas. These nodes have to work with their limited and non replenish able energy resources. Energy efficiency is one of the main design objectives for these sensor networks. In this paper, we present the challenges in the design of the energy efficient medium access control (MAC) protocols for the wireless sensor network. We describe several MAC protocols for the WSNs emphasizing their strength and weakness wherever possible. Finally, we discuss the future research directions in the MAC protocol design.