The Supplementary Guidelines for the Mitigation Function of Defense Teams in Death Penalty Cases (“Supplementary Guidelines”) are the culmination of three years of work coordinated by the Public Interest Litigation Clinic (“PILC”) and the University of MissouriKansas City School of Law in cooperation with seasoned capital litigators and mitigation specialists across the United States. This Article describes the Supplementary Guidelines and the process by which they were researched and developed. Part II discusses the reasons for undertaking this project. Part III describes the process of investigating, researching, and drafting the Supplementary Guidelines. Part IV identifies the scope and goals of the Supplementary Guidelines, and identifies some of the issues that guided our efforts. Part V analyzes the concept of mitigation and its constitutional and practical role in the sentencing process. Part VI explains the central role of the life history investigation in the development of a mitigation case. Part VII discusses the skills and abilities that are essential to the constitutionally effective performance of the mitigation function of capital defense teams. Finally, Part VIII explains the need for capital jurisdictions to provide adequate funding to fully staff capital defense teams.