The living world comprises a dazzling variety of species. Explaining the origins of this diversity is a key goal of evolutionary biology. The process behind diversification is speciation, whereby a single species splits into two or more descendant species. But how can evolutionary biologists study a process that occurs over thousands to millions of years, when the fossil record is often too coarse to follow speciation in detail? The answer is to reconstruct phylogenetic trees, branching diagrams depicting the evolutionary relationships among living species. The growth of molecular phylogenetics, combined with new statistical methods for analyzing the resulting phylogenetic trees, is leading to new understanding of the patterns and processes of speciation.