This thesis is concerned with the exchange of heroin in localised, street-based marketplaces. Commercial exchange of heroin in such sites has been a characteristic of the Australian heroin scene since the early 1990s. Although some qualitative investigations have been undertaken, the dominant approach to understanding these sites in Australia has been quantitative (primarily epidemiological and criminological). These efforts largely adopt a narrow and under-developed conception of 'markets' and much of this work adopts a narrow and circumscribed conception of the subjects who act within these sites. In contrast, this thesis is positioned within a long tradition of ethnographic accounts of drug users as active agents and of drug markets as embedded in particular social, cultural and economic contexts.; In this thesis, I explore two related questions: 1) what are the social relations and processes constituting street-based drug markets, and 2) how do participants in these street-based drug markets express agency, given that, in public and research discourses, they are often understood and depicted either as lacking agency or as expressing agency only through profit-seeking, criminality or both. I explore these questions through an ethnographic examination of the everyday lives of Vietnamese heroin user/dealers who participate in a local heroin marketplace in the Melbourne suburb of Footscray. The key analytical concerns are the social relations through which this particular market is constituted, the social and cultural processes of exchange through which the market is produced and reproduced, and the ways in which participants in the market express agency, including the ways in which their agency might be constrained.; My ethnography of the Footscray drug marketplace reveals that the marketplace is constituted by complex and dynamic social processes and relations. With a focus on drug user/dealers, my analysis condenses to two major themes - those of agency and exchange. Throughout the thesis, I show how, and in what ways, drug marketplace participants act on the world, achieve diverse outcomes (intended or otherwise, constrained or not) and, thus, express their agency. I also demonstrate the complexities of heroin exchange in the marketplace, revealing that heroin is exchanged in multiple ways (e.g. through trade, barter and gifts) for multiple purposes and according to multiple and fluid classifications of social relationships. My account shows the embeddedness of the Footscray drug marketplace - that it is shaped by its particular historical, social, cultural, political and economic context. I show also how market processes - such as exchange - are shaped by culturally patterned ideas about what is right, wrong and even conceivable. This thesis also problematises dominant constructions of drug user subjectivity. Such conceptions have ethical and political implications with regard to the ways in which drug users are understood, judged, regulated and governed. My analysis suggests that the subjectivity of Footscray dealers is ambiguous, contradictory and multiple, constituted not simply by instrumental rationality but by a complex of motivations and by the cultural and social formations which shape these motivations.; This thesis provides an alternative to the dominant approaches to understanding Australian drug markets and marketplaces. Accounts of drug markets tend to privilege an etic view that is theoretically underpinned by neo-classical economic models of markets. Additionally, the quantitative methodological approaches that predominate in Australian drug market research tend to preclude considerations of process and temporality. In contrast, in this thesis I privilege an emic account of the drug marketplace. Influenced by theoretical frameworks drawn from anthropology, in my examination of the everyday lives of drug user/dealers, I stress the importance of the social, political and cultural dimensions of these people's lives and direct attention to the importance and creativity of personal agency.; Drug users and dealers are widely stigmatised and demonised as 'other', juxtaposed against supposedly 'normal' non-drug users. Dominant representations of drug users are unidimensional and do not capture the complexity of drug user agency and subjectivity. This thesis demonstrates that the people who sell heroin in the Footscray marketplace actively engage in a range of exchanges, for a range of purposes - subsistence, the creation of identity, the pursuit of prestige, reciprocity, sociality, the production and reproduction of social relations, and profit-making. My account, therefore, repositions drug users, challenging their stigmatisation by revealing that, in their everyday lives, they struggle with many of the same challenges that confront us all.