Upon completion of their primary treatment, many cancer survivors become “lost in transition,” and receive inadequate or, at best, poorly coordinated follow-up care. Unmet psychosocial and educational needs figure prominently among the concerns identified by survivors of adult-onset cancers in the post-treatment period. This article focuses on the role clinical practice guidelines could play in improving the quality of psychosocial care provided to these post-treatment survivors. After defining clinical practice guidelines and describing their development, the article provides an overview of existing clinical practice guidelines for the psychosocial care of cancer patients and identifies their strengths and weaknesses. A major weakness relevant to this article is that none of the existing guidelines focus on the post-treatment period. Two recent efforts in the field of cancer survivorship are identified that should stimulate and inform the development of guidelines for psychosocial care in the post-treatment period. One effort is the growing movement to implement survivorship care planning at the end of primary treatment. Assessing and addressing unmet and anticipated psychosocial needs have been identified as major components of survivorship care planning. The other effort is the release by the Children's Oncology Group of Long-term Follow-up Guidelines for Survivors of Childhood, Adolescent, and Young Adult Cancers. These guidelines provide a useful model of how guidelines for the psychosocial care of survivors of adult-onset cancers might be developed, organized, and implemented. Cancer 2009;115(18 suppl):4419–29. © 2009 American Cancer Society.