The term 'drug use' covers a wide range of experiences, and drug users are found across all segments of society. For most people who use drugs, the consumption of psychoactive substances is only a small part of their lives. But for a minority of 'problematic' users, it becomes a central aspect of their daily lives and a defining characteristic of who they are. The label 'drug user' does not distinguish across types, but has a tendency to carry negative connotations. This can have important impacts on how drug users are seen - and interacted with - by society (e.g., through stigmatisation and alienation), by the state (e.g., through criminalisation and medicalisation), and by themselves (e.g., through identity formation). Just as drug use itself is a hugely varied and subjective experience, so is the experience of being labelled a drug user. Of course, drug users are only one set of individual actors who make up the drugs landscape. Responses to drug use are also shaped by personal identities and experiences of individual actors - drug policy, for example, is ultimately applied by individual law enforcement officers and treatment professionals. The importance of asking 'who?' in our efforts to understand drug issues extends not just to exploring the variety of individual experiences and identities of users, but also of the individual characteristics of those who interact with them. This addition to the series of books produced by the European Society for Social Drug Research (ESSD) explores the subjectivity behind the label 'drug user'. It is concerned with who people who use drugs are and how their identities are formed, as well as how they are perceived and responded to by a range of different actors. Our contributions draw on empirical work with drug users from across the 'recreational' to 'problematic' spectrum, police officers, and treatment professionals from across Europe. Diverse thought the chapters are in their empirical focus, they address common themes of stigma and normalisation to provide significant insights into the role of identity in shaping drug experiences - and the importance of asking 'who?' as drug researchers.