Ocean waves contain one of the world's largest untapped and predictable renewable energy sources that can be used to fulfil the energy demand in the present energy crises situation. There are many devices that have been proposed and prototyped in different countries all around the world to harness wave energy based on different power take-off (PTO) systems. The aim of this article is to review the power take-off (PTO) systems of the wave energy converters (WEC). The review starts with a brief introduction and background of wave energy. Following this, a novel classification of WEC systems is introduced. Then, the WECs based on the different working methods of their power take off systems are briefly reviewed. This includes an analysis and comparison of advantages and challenges of the power take off systems. Aspects of current international research and development activities and networks for wave energy is also discussed. The current market of wave energy technologies is also assessed, showing that the mechanical direct drive system is the most popular. Hybrid PTO systems are seen as an important development for the future.