Over many years, it has been shown that cancer represents a significant proportion of excess mortality for people with mental illness. In this chapter, we probe this relationship in more detail, and examine the progression of factors that play a role in this finding. Against expectations, people with mental illness are no more likely to develop cancer, even though they have higher exposure to major risk factors including smoking, drug and alcohol use, and obesity. However, even though people with mental illness are just as likely to be diagnosed with cancer, they are more likely to die from it. The reasons for this are multifactorial, including lower rates of routine cancer screening (either because it is not recommended or people with mental illness do not follow through on the recommendation to do so), the increased length of time it takes to be diagnosed after presenting with symptoms, more advanced stage at diagnosis including metastatic cancer at diagnosis, and reduced likelihood of surgical intervention. We discuss the complexities associated with providing medical care for people with comorbid psychiatric disorders and the difficulties faced both by people with mental illness and the people who provide them with medical care.