Despite substantial improvements in the outcomes of patients with cancer over the past two decades, older adults (aged ≥65 years) with cancer are a rapidly increasing population and continue to have worse outcomes than their younger counterparts. Managing cancer in this population can be challenging because of competing health and ageing-related conditions that can influence treatment decision-making and affect outcomes. Geriatric screening tools and comprehensive geriatric assessment can help to identify patients who are most at risk of poor outcomes from cancer treatment and to better allocate treatment for these patients. The use of evidence-based management strategies to optimize geriatric conditions can improve communication and satisfaction between physicians, patients and caregivers as well as clinical outcomes in this population. Clinical trials are currently underway to further determine the effect of geriatric assessment combined with management interventions on cancer outcomes as well as the predictive value of geriatric assessment in the context of treatment with contemporary systemic therapies such as immunotherapies and targeted therapies. In this Review, we summarize the unique challenges of treating older adults with cancer and describe the current guidelines as well as investigational studies underway to improve the outcomes of these patients.