This paper presents a taxonomy of cognitive functions that supports formal functional modeling of cognitive technical systems (CTSs) and cognitive products. To date, there is little support for functional modeling of such systems and products even though their interdisciplinary complexity exceeds that of electro-mechanical products and makes modeling support in conceptual design even more important. The taxonomy of cognitive functions is based on literature research and consists of a set of cognitive capabilities on three hierarchical levels as well as a defined set of flows. Relationships among cognitive capabilities have been identified using WordNet, a lexical database of English. The application of the taxonomy is demonstrated through the example of a coffee robot waiter, which has been designed and prototyped in the research group of the authors. Through defining a common taxonomy of cognitive functions and flows, a common practice for functional modeling of cognitive products is defined thus supporting re-use of functional models. This creates the foundation for creating model-based design repositories for CTSs and cognitive products to support their future development.