While there is a vast philosophical literature exploring the conditions under which it is appropriate to hold individuals morally responsible for their actions, relatively little attention has been paid to the related question of which kinds of individuals merit these responsibility ascriptions. Under normal circumstances, typical adult human beings are held morally responsible for their behaviour but infants and nonhuman animals are not. In this thesis, I aim to account for this difference. That is, I aim to give an analysis (...) of the concept of moral agency. In Chapter One, I begin with a schema of moral agency, under which moral agents are characterised by the possession of certain abilities, enabling certain actions, for which certain responses are warranted. The literature on moral responsibility offers many ways of filling in these details, primarily by specifying the relevant agential abilities in terms of various responsibility conditions. My aim in this chapter is to offer a basic account, under which moral agents are characterised by the simplest possible abilities while still being appropriate targets of the relevant responses, such as praise and blame. I draw on the work of Nomy Arpaly, whose account of moral responsibility identifies the relevant ability as the ability to act out of good or ill will. I offer an analysis of this ability, under which basic moral agency is characterised by the ability to have desires about others’ mental states. In Chapter Two, I consider a range of alternative responsibility conditions offered by other philosophers, each of which appears to be necessary for moral agency. I argue that these conditions are not necessary for a basic account of moral agency. In Chapter Three, I move beyond basic moral agency to offer a more restrictive account that aims to capture other important aspects of our moral lives: the practice of justification and our ability to improve our moral character. I claim these aspects are underpinned by the use of moral reasons to guide our behaviour, and in contrast to moral motivation, this guidance is characterised by the ability to have beliefs about the desirability of actions. In Chapter Four, I aim to answer two questions: at what age do humans become moral agents, and are there any nonhuman animals who are moral agents. I draw on the work of Josef Perner, who offers a three-stage framework of the development of mental representation during childhood. I consider the conceptual coherence of this framework, its applicability to both accounts of moral agency developed in the thesis, and the empirical evidence that bears on the framework. As a result, I tentatively conclude that basic moral agency develops at around 18 months of age and may be present in a few species of nonhuman animal, whereas the more restrictive type of moral agency developed in Chapter Three develops no earlier than around 3.5 years of age and is restricted to human beings. (shrink)