The major function of mitochondria in human cells is to provide ATP by oxidative phosphorylation. However, mitochondria have many other roles including the modulation of intracellular calcium concentration and the regulation of apoptotic cell death. Furthermore, the mitochondrial respiratory chain is a major source of damaging free radicals. Consequently, mitochondrial dysfunction contributes to a number of human diseases, ranging from neurodegenerative diseases and ischaemia–reperfusion injury to obesity and diabetes. In addition, mutations to nuclear or mitochondrial DNA cause a number of human diseases. Therefore, strategies to prevent mitochondrial damage or to manipulate mitochondrial function in clinically useful ways may provide new therapies for a range of human disorders. Here we outline why mitochondria are a potentially important target for drug delivery and discuss how to deliver bioactive molecules selectively to mitochondria within cells.