Founded as a Christian self-help organisation by 60 women and men in St. Gallen in 1899, CSS has since become one of the leading health insurers in Switzerland. The abbreviation CSS stands for "Christian Social Switzerland" and dates back to that time.
In 1947, the number of persons insured with CSS reached 200,000 for the first time; in 1987, the threshold of one million was exceeded. In 2004, CSS acquired the customers and staff of Accorda and founded the Arcosana AG health insurance company. In 2005, Auxilia Kranken-Versicherung AG became part of CSS Group, and three years later CSS Group also acquired INTRAS, with head office in Geneva.