The chapter first considers the nature of ecological theory, then provides an overview of a series of different theoretical ideas in relation to ecosystem collapse and recovery. These include disturbance theory, succession, state-and-transition models, dynamical systems theory, planetary boundaries, critical loads, resilience, food webs, ecological networks and extinction cascades. A set of propositions are then presented that might provide a basis for understanding ecosystem collapse and recovery, based on these theories. These propositions are revisited in subsequent chapters, in light of the empirical evidence that is considered.