PURPOSE: To standardize and improve the safety and efficacy of opioid conversion in people with cancer. METHODS: The Multinational Association of Supportive Care in Cancer (MASCC), American Society of Clinical Oncology (ASCO), American Academy of Hospice and Palliative Medicine (AAHPM), Hospice and Palliative Nurses Association (HPNA), and Network Italiano Cure di Supporto in Oncologia (NICSO) convened an Expert Panel to develop recommendations based on a systematic review of the literature and a formal consensus process. The systematic review focused on randomized and non-randomized studies published from database inception to June 2022. A modified Delphi approach was used to develop and finalize recommendations. Recommendations developed by the Expert Panel underwent two rounds of consensus voting before being finalized. RESULTS: The systematic review, published separately, identified 208 eligible studies. These studies provided mixed and inclusive findings regarding optimal approaches to opioid conversion. In consensus voting, 58 of 84 statements met or exceeded the required 75% level of agreement and were accepted. This process demonstrated some consistencies in conversion ratios between particular opioids internationally, but also uncovered variability in opioid conversion ratios among experts, particularly for methadone. RECOMMENDATIONS: The recommendations address three main topics: pre-conversion assessments, strategies for conversion, and post-conversion assessments. The goal is to reduce the relative risk of overdosing or under-dosing opioids when converting from one opioid to another or converting administration routes. The strength of the evidence from the trials is modest, and there are large clinical practice and research gaps. The panel hopes this guideline will establish an international best practice baseline that can be built upon by new research and better-designed trials. Additional information is available at www.asco.org/supportive-care-guidelines .