This article focuses on how the role and structure of the UK’s Reserve Forces have changed since their foundation before the First World War, with particular attention paid to the last two decades, during which time government has sought to make the Reserves more useable and relevant to post-Cold War military missions, including changing the legislative and administrative basis of their use. Since 9/11, Reserves have played an important role in the defense of the United Kingdom, particularly in operations in Afghanistan and Iraq, but the recent financial crisis has spurred further consideration of how best to structure and use this capability, which has been included as part of the recent Strategic Defence and Security Review (SDSR). The article analyses the current debate on whether the number of Reserve Forces should be cut or increased, and on how best to integrate their efforts with those of the Regular Forces. It also considers the evidence on the recent operational experience of Reserve Forces and its impact on a number of personnel issues, including recruitment, retention, and their health and well-being. The differences between the health and well-being outcomes for Reserve and Regular Forces are discussed and future lines of research enquiry highlighted, while the implications for the comparative analysis of Reserve Forces are also drawn out.