The topic of this chapter is the development of temporal understanding, and in particular the question as to when children can be said to be able to grasp temporal concepts such as 'before' and 'after'. One specific idea we wish to look at is that the development of temporal understanding, and the emergence of a grasp of temporal concepts, is closely linked to developments in children's understanding of causal relationships. There are, of course, substantive theories dealing with the general question of what concepts are, or what it is to possess a concept (see, e.g., Peacocke 1992, Fodor 1998). However, a detailed discussion of these issues is beyond the scope of this paper. Instead, we will start with a fairly rough, intuitive understanding of the explanatory project at hand that locates it somewhere in between two other projects familiar from the philosophical literature on time. One such project is exemplified by attempts to provide what is typically referred to as a causal theory of time. As usually understood, a causal theory of time has it that there is a sense in which temporal notions can be defined in terms of causal notions. Key to causal theories of time such as Reichenbach's (1956, 1957) is the thought that we can give an account of what makes one event a cause and another its effect, rather than vice versa, without using temporal notions. This, in turn, then allows us explain, e.g., what it is for one event to happen before, rather than after, another by reference to the relation in which a cause stands to its effect.