This study explores the role of trade unions in intervening in incidents of workplace bullying, adopting an employment relations perspective. The data was obtained from the members and officials of three major UK unions and analysed using both qualitative and quantitative techniques. Between a quarter and a third of respondents in each union considered they were bullied within a two-year period. A key finding was that most bullied individuals voiced their concerns through non-organisational support mechanisms, including trade unions, in preference to the systems created by employers to address bullying. Colleagues rarely offered overt support and union officials typically responded by providing indirect support to individual bullied members. Outcomes tended to place the onus on perceived targets of bullying to resolve the situation irrespective of the source of support utilised. Employers’ attitudes towards bullying appeared to exert greater influence over resolutions. Whilst unions may have limited power to alter managerially-derived solutions, there was some evidence that, where they engendered a collective response to allegations of bullying, perpetrators were more likely to be held accountable. This study makes a significant contribution to the collective knowledge on workplace bullying by proposing a typology of union responses and an industrial relations model of intervention, which highlights the potential for the responses of unions, co-workers, and employers to affect the balance of power in the employment relationship and influence outcomes of workplace problems like bullying.