The evolution of land plants is characterized by increasing adaptation to the terrestrial environment and increasing structural complexity. Flowering plants have more elaborate structure and life cycles than do mosses, and have diversified and colonized a wider variety of habitats. Nonetheless, analyses suggest that on average flowering plants have neither larger genomes nor more genes than mosses. Genome size varies dramatically among embryophytes but the most significant factor appears to be amplification of transposable elements, which not only mediate genome size but dynamically affect genome structure and function as well. Along with whole genome duplications and lineage-specific expansion of individual gene families, they are responsible for shaping land plant genomes, generating substantial differences in genome size, gene number, and gene order even among closely related cultivars. Comparative analyses of fully sequenced genomes have shed substantial light on the dynamics of genome and gene family evolution; they are just beginning, however, to help us understand the molecular basis of the key adaptations that have allowed plants to thrive on land.