This article presents original research on employer responses to trade union organising campaigns in the United Kingdom. The evidence indicates that there is no single response, with employers in some cases seeking to block and in others support union activity. These different patterns are strongly path dependent and reflect the prior degree of exposure to trade unionism of workplaces targeted for organising. Another finding is that employer responses co-vary with union approaches to organising, such that when the employer adopts adversarial tactics so does the union. The militancy of both parties, it seems, is mutually reinforcing. Finally, the evidence points to substantial influence of employer responses over the outcomes of organising. When employers are supportive then campaigns tend to be more successful, measured on a range of criteria. When the employer is hostile unions find it difficult to make progress and encounter particular difficulties in securing recognition.