The passage of the Safe Drinking Water Act (SWDA) in 1974 (Public Law 93-523) significantly influenced microbiological, chemical and engineering research on drinking water quality in the United States. Microbiological quality research during the past 10 years encompassed the two basic areas of treatment and distribution. Much of the treatment research focused on generation of data to support the evaluation of turbidity and coliform maximum contaminant levels (MCLs) for the Interim Primary Drinking Water Standards. Major effort was also directed toward disinfection research on enteric viruses, pathogenic bacteria and the protozoan pathogen, Giardia lamblia. Basic distribution system studies examined the occurrence of heterotrophic bacterial numbers and types of treated and untreated distribution waters, and coliform occurrence and colonization problems. Methodology developments bridged both treatment and distribution, and included enteric virus detection methods, injured coliform recovery, heterotrophic bacteria enumeration, and Giardia cyst detection. Knowledge gained from the research efforts of the past decade has resulted in new challenges and opportunities to improve drinking water quality.