The authors discuss the plasma physics of comets and in particular the interaction between the cometary ionised environment and the surrounding solar wind. Much of the exotic phenomena observed in comets from the ground, such as the structures seen in the tail and indeed the tail itself, are a result of this interaction. However the cometary plasma environment extends over a much greater volume of space than the visible features indicate. They outline the overall plasma structure and indicate what has been discovered by cometary space missions.