Purpose of Review: Aging is associated with decreased physiologic reserve, and older adults are more susceptible to cancer treatment toxicity. In this review, we discuss the implications of frailty and vulnerability in older patients with cancer. We also review a number of instruments that can be used to assess frailty and vulnerability and propose a practical approach to incorporate these tools in a general oncology or geriatric clinic. Recent Findings: In older patients with cancer, frailty and vulnerability are associated with all-cause mortality, postoperative complications, and treatment intolerance/toxicities. If feasible, a comprehensive geriatric assessment should be used to assess of frailty and vulnerability. If a full geriatric assessment cannot be performed, screening tools such as the Balducci’s criteria, Vulnerable Elders Survey-13, Triage Risk Screening Tool, Groningen Frailty Index, and Geriatric 8 may be used. Summary: Future studies should evaluate geriatric assessment domains that have the greatest predictive value for toxicity for each cancer type and treatment.