Emotions are of increasing interest to the HCI community. Within the last decade, emotion research in HCI grew from an eccentric hobby of some visionary scientists to a widely accepted field of research. A number of proof-of-concept prototypes and studies have been published, dedicated sensor systems and technology frameworks have been developed, and theoretical considerations have been made. While they all represent a very valuable contribution to this young field of research, they lack a common theoretical basis. Particularly, there exists no applicable model of emotions suitable for designing emotion-aware systems or performing HCI-related emotion studies. However, in order to become a mature discipline, emotion research in HCI needs such a rigorous footing that future work can be based on. In this paper, a suitable approach to structure and represent emotions for use in digital systems is introduced, after a detailed and critical review of widely used emotion models is given and representative study results are discussed. The proposed method meets several requirements of HCI researchers and software developers. It avoids artificial categorisation of emotions, requires no naming of emotional states, is language independent, and its implementation is straightforward. The results of an experiment based on this approach are discussed demonstrating its applicability