When a client faces a penalty of death, defense attorneys may call on social workers in many capacities: mitigation specialist, expert witness, consulting specialist, direct witness, or defense-initiated victim outreach worker. The American Bar Association set forth standards for capital defense attorneys, which led an interdisciplinary team to produce the Supplementary Guidelines for the Mitigation Function of Defense Teams in Death Penalty Cases to promote the exceptional competence and diligence required when the consequence is life or death. This article summarizes the Supplementary Guidelines, with implications for social work practicethat is, professional responsibility, competence, interviewing skill, knowledge of behavioral and mental impairment, records review, life history compilation, data interpretation, witness support, law-related knowledge, and testimony. The social work, which is scrutinized in a court of law, requires cultural competence, diverse oral and written communication skills, diligence, and the highest ethical standards.