Oak apples, honeydew and ambrosia galls, witches' brooms, and fasciations - all are types of plant galls, a commonly observed, yet little-understood botanical phenomenon. Often beautiful and bizarre, galls are growths of various shapes, sizes, and colors produced by host plants in response to invading organisms. This guide, a trove of natural history lore, explores this hidden realm, taking a fascinating look at the world of plant galls, the organisms that initiate them, their host plants, and their intricate behaviors. Focusing on native trees and shrubs, but also discussing several galls that occur on herbaceous and ornamental plants, it illuminates the complex interrelationship between botany and entomology and magnifies our awareness of plant communities in the West. This book identifies more than 300 species of galls - 95 on oaks, 22 on members of the rose family, 60 desert species, and 35 species that are new to science. It describes plant galls from coastal dunes, the high Sierra, the Great Basin, forests throughout the western states, and the Mojave and Sonoran deserts. It includes information on host selection, growth and development, predator and parasite defense, and animal and human uses of galls.