There has been substantial scientific debate about the impact of alcohol marketing on consumption. Relying mainly on econometric studies, the alcohol industry has traditionally maintained that alcohol marketing does not influence consumption, but is merely limited to brand level effects. Public health advocates, on the other hand, point to consumer-level research that shows a relationship between exposure to marketing and alcohol consumption, especially amongst the young. Recent longitudinal research has firmly established a causal relationship between alcohol marketing and alcohol consumption, giving the upper hand to the public health critics of alcohol marketing. The new consensus forged by these recent cohort studies has led to two separate, but related, debates. In the first instance, having answered the question of whether marketing influences drinking behaviour, there is a need to establish how and when such effects occur. Secondly, in the face of the mounting longitudinal evidence on the effects of marketing, representatives of the alcohol industry have sought to move the debate away from marketing by explicitly highlighting peer influence as a more significant causal factor in problematic youth alcohol consumption. This thesis tackles both of these new questions simultaneously by harnessing insights developed from social norms theory. An online survey (N = 1,071) was administered to undergraduates of the Dublin Institute of Technology in Ireland, and mediation relationships were tested with logistic and multiple linear regression methods as appropriate. Amongst other findings, the main contributions of this thesis are: (1) that marketing may play a key role in establishing perceived social norms around alcohol consumption, and that these perceived norms may act as an indirect pathway for the influence of marketing on behaviour and (2) that the association between alcohol marketing and consumption may increase as levels of engagement with marketing increase; this engagement appears to be at its most potent when marketing facilitates simultaneous interaction between the consumer, the brand and the consumer’s peers in an online social media environment. This thesis helps to move the field of alcohol marketing scholarship beyond questions of whether marketing influences alcohol consumption to how and when that influence occurs. By showing how peers may act as perpetuators and magnifiers of marketing influence it also undermines the argument that peers matter more than marketing, and suggests that peer norms can act as a powerful marketing tool.