This chapter traces the history and rationale of evolutionary tree building, beginning with Linnaean taxonomy and the shift to Darwinian systematics. Competing schools of systematics—phenetics (numerical taxonomy) and cladistics (phylogenetic systematics)—are compared, showing how reliance on overall similarity vs. shared derived characters leads to different phylogenetic hypotheses. In this regard, homology and homoplasy are compared and contrasted. Furthermore, various kinds of characters and their use in inferring evolutionary relationships, such as synapomorphies, symplesiomorphies and autapomorphies, are discussed. The chapter argues that phylogenies provide more objective, reproducible frameworks than traditional taxonomies, making them essential for modern biology. The chapter ends by comparing and contrasting the use of molecular data vs. morphological data in tree building.