Ensuring the quality of the various processes is essential to the effective functioning of an organisation. One way to achieve this is to assess the quality of the organisation’s performance. This process involves both self-assessment and external evaluation, which are carried out using different methodological approaches to evaluation. It should be stressed that the outcome of the quality assessment also depends on the choice of a methodological approach and its correct use from a methodological perspective. The aim of this paper is to present methodological approaches to assessing the quality of organisational performance. The paper focuses on action research as a methodological approach to evaluation and case studies as a methodological approach to external evaluation, showing how the two are closely interrelated and how they can be used more frequently and successfully in quality management research.