Comparison is an essential research method in political science. This book helps students to understand comparison as an academic instrument, to grasp its necessity and its effective purpose for research. For that reason the author answers three `simple' questions: why compare; what to compare; and, how to compare. Doing so, he considers not only the comparative tradition but he takes also into account methodological innovations of the last two decades. By answering the first question, why compare, the author points out the key reasons for the use of comparison especially for social sciences. He also discusses the objectives of comparison in relation to the problem of generalization, to the difference between explanation and understanding as well as to the theoretical results that were achieved especially in political science. The answer to the second question, what to compare, helps students to understand the requirements for a research issue that can be approached by comparison. It also analyses the basic units of concepts, shows how to operationalize them and how to solve the "many variables, small N" dilemma. Moreover, a focus will be devoted to space and time frames in comparison. The part on, how to compare, depicts and discusses different classical and new approaches to comparison: the so called triangle of Ogden and Richard, the rules of conceptualization, the Tree of Porphyry, the classificatory strategies, the Mill's canons, but also showing the attempt of formalization that took place in this field as well as the recent development with the fuzzy set analysis and the qualitative comparative analysis.