The relevance of Karl R. Popper to the discussion on streams of evaluation knowledge is illustrated through references to Popper in selected works of Donald T. Campbell and Charles C. Lindblom. The two authors hold different positions in regard to theory of knowledge in evaluations, which help to deepen the understanding and clarify points in the actual debate within the evaluation community. The thoughts of Campbell and Lindblom are enriching the discussion of the various points of views in relation to streams of evaluative knowledge and individual evaluation studies that are dealt with through out this book. The relevance of Popper for evaluation is visible on the issue of evaluative knowledge, on use of evaluation, and on unintended consequences of interventions.