This paper examines a discrete set of issues pertaining to the constitution of addiction in law. Based on qualitative interviews undertaken with lawyers in Australia and Canada, I examine how addiction figures in lawyers’ daily practice. Drawing on ideas from science and technology studies scholars Sheila Jasanoff,Michael Lynch and Bruno Latour, and building on recent research I undertook on legal addiction veridiction, I explore the constitution of addiction ‘facts’ in law. I examine how and when lawyers claim to make decisions about addiction in the course of their legal practice. Lawyers report playing a central role in the making of decisions about addiction, at multiple stages of the legal process including: before taking cases on, while running cases in court, and while negotiating and/or settling cases. I argue that these decisions can be properly described as ‘quasi-expert’ determinations with important parallels to scientific, technological and medical claims often made in legal settings by more conventional ‘expert witnesses’. I call these ‘quasi-expert’ decisions because they are decisions of the kind that might be assumed to be the purview of scientific or medical experts and because they have tangible implications for clients. Lawyers uniquely constitute addiction in unique ways, drawing on a combination of factors,including their own experience with and observations about addiction, the experiences of family members who have experienced alcohol and other drug problems, relevant legal concepts and frameworks, popular and scientific claims about addiction, emotions and values, including the gender politics of alcohol and other drug addiction. These addiction ‘facts’ can have a range of material and discursive effects, including potentially adverse implications for people characterised as ‘addicts’. I conclude the paper with a discussion of some implications of these practices, and with reflections on how we might address these issues in future research.