Different approaches are available for the integration of existing health information systems (HIS) in integration projects. Within the PlugIT project in Finland, we have found it necessary to design and implement integration in a collaborative, multidisciplinary and open way. In this paper, we use some generic integration models and relate them to the methods, solutions and experiences of the project. We summarize the results from nine integration teams, methods development and supporting surveys and studies, and discuss these experiences to provide some guidelines for the HIS integration projects in general.