When diagnosed with cancer, patients can immediately make a meaningful positive impact on their health by stopping their tobacco use. Scientific evidence clearly shows that tobacco use in patients with cancer leads to poorer outcomes. The specific biological processes driving tobacco consumption's interference in cancer therapy are the subject of continuing research, but the evidence is clear that tobacco use in patients with cancer leads to decreased treatment efficacy and safety, decreased survival, decreased quality of life, increased treatment-related toxicity, and increased risk of cancer recurrence and second primary tumors. Data suggest that tobacco cessation can improve outcomes and survival in patients with cancer, yet full execution of evidence-based cessation interventions is infrequent in oncology settings. Therefore, both improved provision of cessation assistance to all patients with cancer who use tobacco or have recently quit and further study of the deleterious effects of tobacco use and benefits of tobacco cessation on cancer progression and treatment are needed and recommended by the American Association for Cancer Research. Progress on both fronts begins with universal assessment and documentation of tobacco use as a standard of quality cancer care regardless of treatment setting and will be further facilitated through the development of reliable, valid, and standard measures of tobacco use, incorporation of evidence-based procedures into quality and accreditation procedures, and the development of appropriate training, clinical infrastructure, and incentives for delivery of tobacco cessation interventions.