Adult social life is shaped by conventional practices, and these are often mediated by the use of conventional objects. For instance, we have conventional locations at which to gather for our meetings, conventional ways of cooperating with each other and conventional procedures for conducting formal ceremonies such as weddings. Objects with conventional functions are embedded within these practices, such that we may use train tickets that enable us to travel to those meetings, words to communicate with fellow cooperators, and wedding rings to signal our newly married statuses. This thesis investigates several aspects of children's engagement in, and understanding of social conventional practice. The question of what social conventions are, is complex. Chapter 1 reviews some influential philosophical attempts to tackle the issue, along some particular dimensions of disagreement. Special attention is given to the following questions: whether social conventions are solutions to situations in which people try to coordinate together, whether there is a normative dimension to social conventions, how the notion of 'fiction' relates to conventional phenomena, and what the psychological prerequisites are for understanding and engaging in conventional practice. Chapter 2 reviews existing developmental data on children's understanding of conventionality. This starts with the work of Jean Piaget and his investigation of children's understanding of the underlying structures of social convention through their games. More recent empirical work is then reviewed in which children's understanding of conventionality across the domains of language, tool use and games have been explored. Chapter 3 presents a series of studies in which children's understanding of conventional object functions were investigated. Children (mean age 3;0) played with an object whose pretend identity changed between two different pretend games. They competently tailored their pretend actions to this object when it changed between pretend contexts, showing a grasp of the context-relativity of conventional object functions. The pair of studies presented in Chapter 4, examined children's understanding of the normative aspects of conventional object functions. Children (mean age 3;0) observed a puppet use an object endowed with a pretend identity according to its real function or according to a different pretend identity. They protested when the puppet did this having joined the pretend game but not when he did so outside the game context. This shows a grasp of the way conventional object functions are normatively governed, and a tendency to enforce those normative rules in joint pretence. In Chapter 5, a new study is presented in which children's willingness to adopt a cooperative convention was investigated. Children (mean age 4;9) were engaged in a coordination game in which they could either cooperate with an adult to retrieve some high-value prizes, or act alone to retrieve a low-value prize. It was found that the establishment of joint attention to the high value prizes induced more children to coordinate towards the cooperative convention than did conditions of individual attention. The idea that joint attention may operate as a developmentally primitive form of 'mutual knowledge' in children, enabling coordination is discussed. Chapter 6 summarizes the results and theoretical implications of these studies, and highlights directions for future research.