Systematic biogeography, the discovery of classification of regions based on biotic area homologies, is a newly emerging method of biogeography that bridges phylogenetic and distribution patterns. This chapter reviews the history of area classification as it relates to the development of comparative biogeography, and introduces the notion of area relationship, known as area homology, which forms the basis of geographical congruence or area monophyly. The chapter focuses on the application of a systematic biogeography to demonstrate biotic area homology, and then explores mechanisms or processes that may have given rise to general patterns or expressions of area relationships.