Modern oncology practice is built upon the idea that a patient with cancer has the legal and ethical right to make decisions about their medical care. There are situations in which patients might no longer be fully able to make decisions on their own behalf, however, and some patients never were able to do so. In such cases, it is critical to be aware of how to determine if a patient has the ability to make medical decisions and what should be done if they do not. In this article, we examine the concept of decision-making capacity in oncology and explore situations in which patients may have altered/diminished capacity (e.g., depression, cognitive impairment, delirium, brain tumor, brain metastases, etc.) or never had decisional capacity (e.g., minor children or developmentally disabled adults). We describe fundamental principles to consider when caring for a patient with cancer who lacks decisional capacity. We then introduce strategies for capacity assessment and discuss how clinicians might navigate scenarios in which their patients could lack capacity to make decisions about their cancer care. Finally, we explore ways in which pediatric and medical oncology can learn from one another with regard to these challenging situations.