Castro Valley is a census-designated place (CDP) in Alameda County, California, United States. As of the 2000 census, it is the fifth most populous unincorporated area in California, and the twenty-third in the United States. The population was 61,388 at the 2010 census.
Castro Valley is served by the Castro Valley Unified School District. Overall, the district contains almost 9,000 students. The main high school is Castro Valley High School with over 3000 students. Castro Valley also has Redwood High School, an alternative high school with approximately 193 students in 2005. The school district includes the Castro Valley Adult School.
There is also a Roman Catholic school, called Our Lady of Grace (K-8), which is part of the Roman Catholic Diocese of Oakland. Redwood Christian Schools has one elementary school (K-5) Redwood Christian Elementary.
Interstate 580, which approaches from the east, makes a turn northward at Castro Valley. Interstate 238, which originates in Castro Valley, connects I-580 to Interstate 880. In addition to being served by those two freeways, Castro Valley is served with public transportation by bus system AC Transit, and rapid transit system BART with a station.
Historically, Castro Valley Boulevard was part of the first transcontinental highway system, the Lincoln Highway.