Comparative politics involves the systematic study and comparison of the world's political systems. It is systematic in that it looks for patterns, regularities, and trends among all these political systems; it is comparative in seeking to explain similarities, differences, as well as developmental changes among and between these systems. By the 1920s and 1930s, comparative politics had developed as one of the major fields within political science. The comparative politics field was strongly influenced by refugees from Europe during the 1930s and 1940s who themselves had been trained in constitutional law. Roy Macridis had been one of the "young Turks" of the post-World War II generation of comparativists who had been advocating change and new directions. In the late 1950s and early 1960s, a host of newly independent nations emerged onto the world stage in Africa, Asia, the Middle East, and the Caribbean. The chapter also presents an overview of the key concepts discussed in this book.