Conflict occurs at a variety of scales, from violent clashes between nations to a difference of opinion between two individuals. The focus of this book is those small-scale, non-violent conflicts which are much more common, insidious and problematic for sustainable development. This chapter defines the notion of conflict and explores how existing theories can help us to understand marine and coastal conflict. Work by early social conflict theorists and later environmental (and natural resource) conflict theorists is reviewed, and the question of if, and how, these theories address the peculiar characteristics of the marine and coastal environment identified in the previous chapter is considered.