Originally published in 1986. The theory of events presented is one that construes events to be concrete particulars; and it embodies an attempt to take seriously the idea that events are the changes that objects undergo when they change. The theory is about what an event really is, about when events are identical, about what properties events have essentially, and about what relations events bear to entities of other kinds. In addition, this book contains an account of what philosophers are up to when they provide reasons for thinking that objects belonging to metaphysically interesting kinds exist. It also gives an account of the role of criteria of identity (eg. identical sets must have the same members) in such reasons, and an account of what criteria of identity must be like in order for them to be able to play such a role.