In previous chapters the potential for harm to mental health through social processes and social relationships was discussed. This chapter discusses how teenagers' peer networks which develop within their community are the source of their drug use. The peer relationships that teenagers form, develop and change over time providing the teenagers with continually changing peer networks. Drug use is introduced to those peer networks when one member takes drugs. It is diffused throughout the peer network when that individual gives the drug and social support for its use to other members of the network or when the individual provides social support to others members of the network for the use of drugs which they have procured elsewhere. Changes in peer relationships over time facilitate the continued diffusion of drugs within the peer networks and between peer networks. Social support in this context contributes to the initiation and continuation of a deviant behaviour and could, therefore, be considered to have a negative effect. The social support of their teenage peers plays an important part, however, in the normal psychosocial development of teenagers (see also Champion, this volume). It would seem important that clinicians take account of the importance of teenagers' peer networks in dealing with the source and treatment of their individual psychosocial and social crises. This could most usefully be done by examining the teenagers' peer networks in the concrete way in which they are examined in this chapter, rather than dealing with the abstract concepts of peers, peer group and peer influence as has traditionally occurred.