New Public Governance assumes that co-production leads to beneficial outcomes, such as increased efficiency and better citizen well-being. However, few empirical studies have documented these outcomes, and some have demonstrated that the assumed outcomes do not emerge. This study establishes that co-production is a complex, social phenomenon, which implies that there cannot be a clear cause-effect relationship between co-production activities and their outcomes. To qualify and enable further empirical investigation of the outcomes of co-production, the study proposes that contribution analysis should be applied as an appropriate evaluation paradigm to theoretically reduce complexity and define a generic programme theory for co-production. The study also discusses how the creation and operationalisation of a local coproduction programme theory can take place to evaluate the relationship between co-production initiatives and outcomes on a localised level. Finally, directions are provided for how the outcomes of co-production can be coevaluated with citizens.