ISA² (Interoperability Solution for public administrations, businesses and citizens) was a spending programme of the European Union (EU) that supported the development of digital solutions, which enable public administrations, businesses and citizens in Europe to benefit from interoperable cross-border and cross-sector public services. With a budget of €131 million, ISA² was one of the main instruments supporting and promoting public sector interoperability between 2016 and 2020. This independent Study supports the final evaluation of the programme, thus helping fulfil the legal obligation of evaluating the performance of the programme, by 31 December 2021, stipulated in the ISA2 Decision. In addition, the Study helps inform future measures in the field of interoperability, particularly the implementation of the Digital Europe Programme and the development of a new interoperability policy for the EU’s public sector. The Study was performed between September 2020 and June 2021 and considered seven evaluation criteria: relevance, effectiveness, efficiency, coherence, EU added value, utility and sustainability. The analysis in this Study is based on primary data collected from 102 stakeholders who replied to the targeted or public consultation, as well as secondary data such as publicly available sources (e.g. the ISA2 website and Joinup), operational documents of the programme, relevant policy documents and relevant literature in the field of interoperability. In addition, five independent expert assessments complemented the analysis.