The Santa Ana River is the largest watercourse in the heavily populated coastal plain of southern California and is home to over 800 plant species. Despite damaging environmental incursion into this region, the Santa Ana River remains an abundant area of biological diversity. With over 2000 illustrations, the Santa Ana River Flora: A Phylogenetic Approach is a comprehensive field guide to the flora of the Santa Ana River. Three hundred prominent plant species are described in basic botanical concepts and relationships to prepare the reader for further interpretation of natural history.