The purpose of this project was to describe each of the native and naturalized vegetation types known to occur within western San Diego County and to provide the user a means to determine each type through direct observations of species composition. The classification presented herein is the result of a detailed analysis of data collected throughout the western San Diego County study area. Under contract to the San Diego Association of Governments (SANDAG), Biologists from AECOM, Conservation Biology Institute, and the California Department of Fish and Game (CDFG) Vegetation Classification and Mapping Program (VegCAMP) collaborated on these analyses, the definition of the classifications, and preparation of this manual. This classification study was conducted in a manner consistent with the recommendations for standardized data collection and analysis by CDFG VegCAMP (https://wildlife.ca.gov/Data/VegCAMP) and the methods used in the preparation of A Manual of California Vegetation, 2nd ed. (Sawyer; Keeler-Wolf; Evens 2009), published by the California Native Plant Society (CNPS). This projects classifications is in accordance with this larger work. A Manual of California Vegetation (MCV) is intentionally consistent within the larger context of the National Vegetation Classification System (NVCS), which has been adopted by federal agencies and nongovernmental organizations such as the US Geological Survey, National Park Service, and NatureServe. Thus each of these classifications can be compared in context with the others nationwide.