This chapter explores the principles of comparative research design as well as the issues and problems associated with different aspects of the approach. In particular, it considers the issue of case selection, the common sources of error that are associated with comparative research, and what can be done to try and avoid or minimize them. The comparative method is one of the most commonly used methods in political research and is often employed to investigate various political phenomena, including democratization, civil war, and public policy. The chapter discusses the three main forms of comparison, namely case study, small-N comparison, and large-N comparison. It also describes two main approaches used to select cases for small-N studies: Most Similar Systems Design and Most Different Systems Design. It also evaluates qualitative comparative analysis and concludes with an analysis of issues arising from case selection and data collection in large-N comparative research.