Modern groupware allows us to work together, while bridging differences in space and time. However, there exists no standard to design groupware, which has resulted in a lack of conceptual consistency and interoperability. Additionally, current groupware design methodologies typically focus on implementation details. In contrast, we consider the service a groupware application provides to be the most important aspect of the system. Dynamic Groupware Services provides a structuring of groupware services. This structuring allows co-operating end users to select and compose groupware modules in order to form groupware services that match their needs. At the same time, this structuring helps designers design tailorable groupware services. Two important results of this structuring are a description of the elementary units of groupware behaviour and a description of the units to compose groupware services. The latter units, as well as the relations between them, are defined in a reference model for tailorable groupware services.