The concept of 'trigger' is a core concept of Chomsky's Minimalist Program. The idea that certain types of movement are triggered by some property of the target position is at least as old as the notion that the movement of noun phrases to the subject position is triggered by their need to receive nominative case. In more recent versions of syntactic theory, triggering mechanisms are thought to regulate all of movement. Furthermore, a quite narrow range of triggering mechanisms is permitted. As is to be expected, such a restrictive approach meets a variety of difficulties. Specifically, the question is whether all triggering elements required to cover displacement of all kinds in natural language can be independently motivated. Further, how...