This paper presents findings from a study of the context and processes of provider mergers in the NHS in England. Mergers are an example of organisational restructuring, a key lever for change in the UK health care sector and elsewhere, although it is only one strategy for organisational change. The framework for the study is key themes from the organisational change literature: the complexity of the effects of change; the importance of context; and the role of organisational culture. The drivers for health care mergers and the evidence for these are analysed. Using documentary analysis and in-depth qualitative interviews with internal and external stakeholders, the first part of the paper reports on stated and unstated drivers in nine mergers. This provides the context for four in-depth case studies of the process of merger in the second and third years post-merger. Our study shows that the contexts of mergers, including drivers of change, are important. Merger is a process without clear boundaries, and this study shows problems persisting into the third year post-merger. Loss of management control and focus led to delays in service developments. Difficulties in the merger process included perceived differences in organisational culture and perceptions of 'takeover' which limited sharing of 'good practice' across newly merged organisations. Merger policy was based on simplistic assumptions about processes of organisational change that do not take into account the dynamic relationship between the organisation and its context and between the organisation and individuals within it. Understanding the process of merger better should lead to a more cautious approach to the likely gains, provide understanding of the problems that are likely in the period of change, and anticipate and avoid harmful consequences.