The California Floristic Province (CA-FP) encompasses North America's Mediterranean-type climatic region and is a global biodiversity hotspot in temperate North America. Fine-scale phylogenetic studies and increasing scrutiny of morphological, ecological, and geographic variation in the CA-FP vascular flora continue to reveal novel, often cryptic diversity warranting taxonomic attention. The prevalence of such discoveries warrants caution about potential for loss of undescribed or unrecognized biodiversity from human-related impacts, including insufficiently informed conservation-related actions (e.g., genetic augmentation or assisted migration). Broader-scale molecular phylogenetic studies have yielded other examples of more extensive CA-FP diversification than previously resolved. For example, clades uniting CA-FP taxa not earlier thought to be such close relatives have been discovered in a wide diversity of tribes of composites and legumes. This understanding adds additional weight to the conclusions of Raven and Axelrod (1978) that in situ evolution, especially since the pronounced mid-Miocene transition toward summer-drying, has been a major factor in the origins of Californian vascular plant diversity. The importance of paleo-endemism in gymnosperms and (especially woody) angiosperms of the CA-FP flora also has been corroborated and refined by molecular phylogenetic studies, with improved understanding of the timing of divergence and relationships of isolated lineages, such as the extinct island-endemic Hesperelaea A. Gray (Oleaceae). On a regional scale, studies of spatial patterns of Californian species richness, phylogenetic diversity, and phylogenetic endemism by the California Plant Phylodiversity Project (CPPP) have reinforced the floristic importance of areas of high topographic complexity, or environmental heterogeneity more generally, where relatively high levels of habitat diversity, speciation, and lineage persistence may be expected. The CPPP's finding that particularly young plant lineages and significantly high phylogenetic endemism are concentrated in drier regions of the CA-FP and in the adjacent Californian deserts has corroborated earlier preliminary findings and aligns with Stebbins's (1952) hypothesis of aridity as an evolutionary stimulus. A recent conservation gap analysis by the CPPP incorporating flora-wide phylogenetic and geographic occurrence data plus land-protection status and habitat-integrity information for California has demonstrated the potential of an integrative, evolutionary approach for identifying high-priority land conservation targets.