"This publication is based on the experiences collected over the course of 30 months while implementing participatory evaluation practices in the European funded research project CoAct. This citizen social science project's primary goal was to address social concerns such as youth employment, mental healthcare, environmental justice and gender equality in the context of local citizen social science initiatives [...] With this Whitepaper, we want to raise awareness for participatory approaches towards evaluation and impact assessment in citizen social science. The six co-evaluation principles that form the core of this paper are intended to guide the participatory approach to project evaluation and to sharpen the focus for impact assessment. While these principles have been developed in the context of citizen social science activities, we believe in their wider applicability for citizen science in other domains and participatory research in general." (Introduction, page 7)