The extramarital sexual underground is defined by its prevalence and by its secret nature, even to the degree that marriage counselors may not be informed that an extramarital affair has occurred or is occurring. Reasons for affairs and for secrecy are provided, and the identifying characteristics of extramarital affairs are described. The authors present ethical methods that can be used by marriage counselors to promote revelation ofsecret andpotentially detrimental extramarital affairs. A strict ethical and legal position is taken regarding counselor responsibilities.