A survey is given covering synchronization techniques for large-capacity time-division multiple-access (TDMA) systems using geosynchronous satellites. Five important classes are identified and compared. Also, several methods of initial acquisition are described. The fundamentals of multiple access and TDMA in particular are presented. Summary descriptions and comparisions of the TDMA systems developed to date are given for experimental, prototype, and operational systems. Some important related technologies are introduced. It is concluded that TDMA is in a maturing period.