The Peninsula Watershed has been integral to the story of San Francisco's growth ever since the Gold Rush. The rapid influx of settlers to San Francisco during the Gold Rush spurred a sudden demand for a reliable water source, which led to the formation of the Spring Valley Water Works (later purchased by the Spring Valley Water Company [SVWC]) in 1858 (Hanson 2005 ). Over the subsequent 70 years, SVWC bought up large swaths of land on the Peninsula, and constructed a complex system of dams, tunnels, and pipes to capture and transport water to San Francisco. Within the Peninsula Watershed, this system includes the Crystal Springs and San Andreas reservoirs, located in the San Andreas Creek, Laguna Creek, and Upper San Mateo Creek basins along the San Andreas Fault; and Pilarcitos Reservoir the west, located in the Pilarcitos Creek Basin along the Pilarcitos Fault. The City of San Francisco purchased SVWC in 1930, and today the Peninsula Watershed, managed by the San Francisco Public Utilities Commission (SFPUC), continues to be a key source of water for San Francisco and for other communities in the South and East Bay. Despite the past 150 years of reservoir construction and other hydrologic modifications, the construction of transportation and utility corridors, and the large-scale suburban development that has occurred to the east, the Peninsula Watershed has remained largely undeveloped and is managed to protect water quality, water supply, wildlife habitat, and a range of other natural and cultural resources. The watershed supports some of the largest intact remnants of contiguous habitat in the region, including extensive oak woodlands, old-growth Douglas-fir forests, serpentine grasslands, chaparral, and coastal scrub. Over the past 250 years since Spanish explorers first set foot on the watershed, however, changes in disturbance regimes and other large-scale anthropogenic modifications, including fire suppression, homesteading, livestock grazing, agriculture, tree planting, introduction of plant pathogens, spread of invasive species, and climate change, have altered vegetation dynamics and changed the distribution and structure of vegetation communities throughout the watershed. The changes have raised many questions about the historical ecology of the watershed: What was the extent, distribution, and composition of terrestrial, riparian, and wetland habitats prior to Euro-American modification? How have vegetation distributions changed over the past two centuries, and what are the implications of those changes for species support? Are there remnant patches of relatively unmodified habitat present in the watershed, or areas that are currently in a state of recovery? Where are current habitat characteristics most similar to or different from historically documented conditions? How have key natural and anthropogenic disturbance regimes and processes changed over time? The Peninsula Watershed Historical Ecology Study aims to advance understanding of landscape conditions of the Peninsula Watershed prior to major Euro-American modification, and to provide insights into the nature and drivers of vegetation change since the first Spanish explorers set foot in the watershed 250 years ago. The primary goal of the research was to examine the historical extent, distribution, and composition of terrestrial vegetation types and their trajectories of change within the watershed. To the extent possible, research also addressed historical riparian, wetland, and estuarine habitats; hydrology and sediment dynamics; wildlife support; land use history; and a range of other topics.