Affect-adaptive games gained in popularity over the last years in human computer interactions studies, promising potential benefits for player experience, performance, and even health. It is however not yet clear how affective games are being evaluated, what the precise effects are, and how they are based on emotion theoretical concepts that are still not universally agreed upon. This systematic review investigated these questions by analysing relevant high-quality evaluation studies of the effect of affect-adaptive video games on various outcomes in regards to their effects, theoretical assumptions, and methodologies. Out of 3,930 papers, 26 studies were included based on preregistered inclusion and exclusion criteria. A high variance regarding theoretical assumptions and methodological approaches was observed, as well as an overall poor methodological rigour, leading to the conclusion that more work is needed in constructing better methodological standards for game evaluation studies and theoretical considerations when developing and testing affect-adaptive video games.