The Billings/ovulation method is a periodic abstinence method of regulating births based on the client's interpretation of changing patterns in secretions of cervical mucus monitored by external self-examination. It was developed in Australia and is now widely promoted overseas. This paper outlines the method's recent history and goes on to discuss its physiological basis, its use-effectiveness as measured in a number of major trials, and some evidence concerning its general acceptability and applicability in family planning programs.