Over the last two decades, the healthcare sector of different Countries has been involved in a wave of mergers. This wave first started in the US and in the UK and, later, has involved Italy. Despite these activities are still pursued, little attention is paid to the assessment of their impact. The aim of this study is to summarize, through a review of the literature, evidence on the impact of these mergers. The main results are divided into four areas: the impact of mergers on clinical outcome, processes and use of resources; the association between population size and performance of primary care organizations; the analysis of the main drivers; and eventually, staff perception and satisfaction. Evidence is conflicting and small number of indicators of clinical outcome, processes and use of resources show a significant improvement. The performance of the Primary care Organizations does not seem significantly related to the size of the population served but to numerous factors, among others the function of the organization itself. Drivers that lead the pursuing of merging can be distinguished in stated and unstated drivers the first ones originating from public consultation document, the second ones from staff interview. Concerns about mergers derive from responses of personnel interview, in particular about the cultural differences and the distance perceived from top managers. Evidence shows that these processes do not necessarily lead to the expected benefits. It is important, therefore, to periodically and systematically assess the impact of mergers in a continuous quality improvement cycle that makes professionist, recipients and policy maker accountable. These latter, in particular are responsible for protecting the health of the community.