This chapter examines the influence of employer decision-making and behaviour on union organization. A simple adversarial view of the employment relationship may lead us to expect that this influence would be predominantly negative; as the opponents of trade unions, employers might be expected to engage in a variety of preventative activities ranging from the aggressive – for example victimization of activists – to what Kochan (1980) termed ‘substitution’ – namely the provision of a work environment removing the perceived need for representation. However, until recently, UK employers’ decision to recognize unions was purely voluntary (see Moore, this volume). In addition, employers provide time and facilities for trade union representatives and check-off mechanisms for trade union subscriptions which, whether conceded by bargaining pressure or not, support union activity. Employers also take other decisions on matters such as where to locate, how to deploy labour, and how much labour to deploy which, while not perhaps directly about industrial relations matters, will influence the contours of union organization. In this chapter, we focus primarily on employer decisions about trade union matters although, as we shall show, unionization is affected by compositional change among employers.