Aging is a heterogeneous process. Most newly diagnosed cancers occur in older adults, and it is important to understand a patient's underlying health status when making treatment decisions. A geriatric assessment provides a detailed evaluation of medical, psychosocial, and functional problems in older patients with cancer. Specifically, it can identify areas of vulnerability, predict survival and toxicity, assist in clinical treatment decisions, and guide interventions in routine oncology practice; however, the uptake is hampered by limitations in both time and resources, as well as by a lack of expert interpretation. In this review, we describe the utility of geriatric assessment by using an illustrative case and provide a practical approach to geriatric assessment in oncology.