Mitochondria have traditionally been viewed as the powerhouse of the cell, where they serve, amongst other functions, as a major source of ATP generation. More recently, mitochondria have also been shown to have active roles in a variety of other processes, including apoptotic cell death and inflammation. Here we review the various ways in which mitochondrial functions affect cancer. Although there are many diverse types of cancer, hallmarks have been defined that are applicable to most cancer types. We provide an overview of how mitochondrial functions affect some of these hallmarks, which include evasion of cell death, de-regulated bioenergetics, genome instability, tumour-promoting inflammation and metastasis. In addition to discussing the underlying mitochondrial roles in each of these processes, we also highlight the considerable potential of targeting mitochondrial functions to improve cancer treatment.