Horizon Services, Inc. (HSI) is a nonprofit social service organization providing alcohol and drug treatment and recovery, mental health recovery, and substance abuse prevention services in Alameda, San Mateo and Santa Clara counties in the San Francisco Bay Area of California. The executive director is Keith Lewis, who has held the position since 1986. Their programs utilize the Alcoholics Anonymous and Narcotics Anonymous models, along with other methods of treatment. HSI is based in Hayward, California, which is a source for some of their funding.