A case study is an in-depth examination, often undertaken over time, of a single case – such as a policy, programme, intervention site, implementation process or participant. Comparative case studies cover two or more cases in a way that produces more generalizable knowledge about causal questions – how and why particular programmes or policies work or fail to work. Comparative case studies are undertaken over time and emphasize comparison within and across contexts. Comparative case studies may be selected when it is not feasible to undertake an experimental design and/or when there is a need to understand and explain how features within the context influence the success of programme or policy initiatives. This information is valuable in tailoring interventions to support the achievement of intended outcomes.