This report presents findings from the Santa Clara County (California) survey of Drug, Alcohol, and Tobacco Use among Students in Grades 5, 7, 9, and 11 administered during the spring of 1991 to 5,180 students in 51 randomly selected county schools. An executive summary discusses sampling error, sample demographics, and findings on drug use prevalence. The main text of the report contains sections on the background of the project, the methodology used, and results and conclusions. Results show that reported alcohol and marijuana use levels were lower than those found in the most recent state survey for grades included in both surveys (grades 7, 9, and 11). For other illegal drug use, the findings discussed show that the Santa Clara sample's reported use was slightly higher that the state's reported use; state rates were higher for grades 9 and 11. No county-state comparisons regarding tobacco use were possible. For grades 7, 9, and 11, strong relationships are noted among some environmental risk factor scales and drug use within the three main categories of tobacco, alcohol, and marijuana. Seventy-three tables, 27 figures, and references are included. Information on reliability and validity and on sampling error estimation methods are contained in a technical appendix. Survey forms and instruments, letters, pilot materials, administration instructions, scale definitions, and a list of participants are appended. (NB)