About 20 percent of those jailed each year are mentally ill or developmentally disabled; in addition, about 120,000 are juveniles under the age of 18, for whom community-based alternatives have not been developed or appropriately used. One of every three arrests is for public intoxication, and others are incarcerated for problem behaviors associated with drug use. Jail populations have an array of unmet needs, including insufficient resources and discriminatory structures within the community. Service deficiencies in jails may increase the likelihood that the inmates will leave the jails in poorer mental and physical condition than when they entered; for example, medical and dental care may be judiciously meted out, while unattended infectious diseases and illnesses take their toll. Other problems, including self-mutilation, interpersonal violence, and inmate suicide, are widespread in prisons. Jail conditions and the impact of incarceration on inmates and families have implications for social work intervention in jails. Counseling, referral, and crisis intervention are social work services, and are often the forerunners of long-term jail policies and practices. Social workers can also provide informal training in interpersonal skills for custody staff. Thirty-five references are included.