Public Participation GIS (PPGIS) is a field of research that, among other things, focuses on the use of GIS by non-experts and occasional users. These users tend to have a diverse range of computer literacy, world views, cultural backgrounds and knowledge. These aspects require that the systems used within PPGIS are accessible and easy to use. Human-Computer Interaction (HCI) and the related usability evaluation techniques focus on how to make computer systems more accessible, while focusing on user needs and requirements. Thus, the synergy between PPGIS and HCI seems natural. In this paper, we discuss the aspects of this synergy, building on our experience from three workshops. We demonstrate how usability evaluation can contribute to PPGIS research, and how PPGIS research can contribute to the HCI aspects of GIS in general. We conclude this paper with a call for a user-centred design approach to PPGIS projects.