As the worldwide population ages, oncologists are often required to make difficult and complex decisions regarding the treatment of older people (aged 65 years and older) with cancer. Chronological age alone is often a poor indicator of the physiological and functional status of older adults, and thus should not be the main factor guiding treatment decisions in oncology. By contrast, a geriatric assessment can provide a much more comprehensive understanding of the functional and physiological age of an older person with cancer. The geriatric assessment is a multidimensional tool that evaluates several domains, including physical function, cognition, nutrition, comorbidities, psychological status, and social support. In this Series paper, we discuss the use of a geriatric assessment-based approach to cancer care, and provide clinicians with tools to better assess the risks and benefits of treatment to engage in shared decision making and provide better personalised care for older people with cancer.