Governments everywhere are increasingly interested in assessing the effects of their policies and the effectiveness of public institutions, and competition agencies are no exception. This Reference Guide contains an in-depth overview of all the issues linked to ex-post assessments and contains numerous examples and references. It constitutes an excellent resource both for the competition authorities who are planning to start performing ex-post evaluations and for those who already do it but want to improve the quality of their assessment. The Reference Guide complements a Guide for assessing the impact of competition authorities' activities (2014) and a Factsheet on how competition policy affects macro-economic outcomes (2014).