The construction kit principle is a well-known software engineering paradigm to foster reusability. In case the construction kit principle is applied at runtime it is even a way to implement flexibility. In today’s workflow technology the construction kit principle is applied, e.g. in in hierarchical modeling approaches using subprocesses. In this paper we propose a construction kit application based on process fragment. In contrast to subprocesses, process fragments represent non-complete process knowledge, which needs to be integrated with further process knowledge to become a complete process model. Integrating one process fragment with another process fragment requires complex composition operations, since process fragments do not represent the implementation of a single abstract activity like in subprocesses, but are knitted together on the same level of granularity. The advantage of process fragments lies in their means to represent noncomplete process knowledge. In this paper we propose a formal process fragment modeling language, which is based on current workflow standard languages, like BPMN. Based on this modeling language we design a basic operation set, which allows to compose process fragments.