The effectiveness of the measures implemented under the EU cohesion policy has become a significant issue. The relevance of the topic is undeniable, given the ongoing discussion of the results of public interventions conducted at the EU level, and the resulting concept of an integrated approach used to study the effects of actions undertaken within the cohesion policy. Based on various studies conducted to date, the authors argue that the effectiveness of the funds allocated in Polish regions could be greater if we focused more on complementarity of the undertaken actions. The paper concentrates on issues related to complementarity between programmes and projects implemented under the EU Cohesion Policy in Polish regions. A case study of a Polish region – namely, Łódzkie – shows that the adopted methods and tools, intended to ensure complementarity, do not work in a proper way. The results suggest the existence of severe restrictions of assessing the actual scope of complementarity and the resulting synergy effects. As both analyses of documentation and in-depth participant interviews indicate that the concept of complementarity in the projects is misunderstood or deliberately distorted (so that the highest score during the application stage can be obtained), it can be stated that the scale of complementarity is far from satisfactory. Based on their research, the authors make conclusions and recommendations regarding the solutions that should be taken into account in order to improve the implementation of the complementarity concept in the next financial perspective for the years 2014–2020.
