This report describes the unique preschool participation patterns and demographic characteristics of low-income families in San Mateo and Santa Clara Counties, focusing on characteristics that could help identify which barriers most often prevent preschool participation. In addition, we examine the geographic distribution of preschool enrollment, focusing on areas with the lowest preschool participation. This report is one of two studies conducted by the Urban Institute to examine preschool participation in Silicon Valley. The companion report explores the barriers to early care and education low-income children of immigrants face in Silicon Valley and identifies possible strategies to address them.