BACKGROUND: Existing evidence demonstrates the benefits of integrated palliative care for people with cancer, for improved symptom burden, quality of life for patient and caregiver, and appropriate healthcare resource use. The integration of palliative care and oncology has the potential to reduce suffering and is recommended by international guidelines. However, it is not yet consistent practice. There are many approaches to integration, but it is unclear what works, for whom, and in what contexts, to achieve the best possible outcomes for patients, families, and healthcare systems. METHODS: Realist review, conducted in accordance with RAMESES quality standards. Evidence was identified through systematic academic databases searches and stakeholder engagement. Data were extracted from included articles and synthesized using realist analysis to develop explanations of how and why integrated palliative care in oncology works, for whom, and in what contexts. RESULTS: One hundred sixty-four papers were included in the review, from 33 countries, and involving a range of inpatient, outpatient, and home-based care settings. Integrated palliative care and oncology could improve patient outcomes, increase the goal-concordance of patient care, and support workforce wellbeing. Interventions towards integration should be tailored to the context in which they are delivered. Ensuring the timely delivery of palliative care for people with cancer requires integration that overcomes siloes between oncology, specialist palliative care, and primary and community care. The motivation to prioritise the integration of palliative care relies upon all stakeholders first understanding its value. Enriched interdisciplinary collaboration involves developing staff skills and confidence, facilitating coordination between care settings, and supporting communication within and between teams. Leadership is needed at all levels to attend to the structural and social norms of care. CONCLUSIONS: The success of integration is influenced by the ways in which palliative care is understood, prioritised, operationalised, and measured within oncology. Through the synthesis of international evidence, this project draws on implementation science to contribute clarity on how integrated palliative care and cancer care can be achieved in practice.