In wireless sensor network, medium access control (MAC) has been at the core of effective communication. Since the traditional MAC layer protocols don't adapt themselves to the performance traits and technique request of wireless sensor network, many MAC protocols for wireless sensor network are studied. Design principles and classification methods for MAC protocols in wireless sensor network are summarized, and fundamental mechanism of each recent representative MAC protocol is analyzed in detail. The characteristics, performance, and application areas of various MAC protocols in wireless sensor network are adequately compared. Finally, the status of current research development are concluded and the open research issues on MAC layer design are pointed out.