An Expertise Management ontology (EMont) is presented with which human expertise is captured. EMont aims at describing and collectively finding improvements for complex situations in which people interact with often conflicting worldviews in order to achieve goals, shared or not. EMont is rooted in systems thinking, in particular Soft Systems Methodology, to be able to accommodate stakeholders’ worldviews and activities in complex situations. Its purpose is to model human activity systems, which is illustrated with the help of a complex situation. EMont forms the heart of the Expertise Management Methodology (EMM). This methodology can be regarded as a framework to build bodies of knowledge for particular domains systematically. EMM favors a research process of abduction. By studying one or more situations, generalized models are developed that explain the observed phenomenon best. The models are then taken as a starting point to conduct additional studies. This cyclic process is repeated thereby strengthening the models. EMont has been applied in several, diverse domains such as flood protection, health care and community resilience, over a period of more than four years. In our experience, EMont is a stable ontology, well-suited to capture human activities and identify widely supported solutions for complex situations, and has proven its value in practice. EMont has been implemented in Semantic Media Wiki to publish bodies of knowledge on the web.