Functional near-infrared spectroscopy (fNIRS) is commonly used in studies of cerebral blood oxygenation and tissue hemodynamics. In recent years, fNIRS has awakened interest also as a potential tool for cancer diagnostics, particularly for breast cancer. Although clinical applications of fNIRS are still missing, they are anticipated to emerge because of fNIRS’ capability to detect microvasculature and blood oxygenation concentration related parameters, which in tumours often differ from those of healthy tissue. Furhermore, since fNIRS offers a broad range of contrast mechanisms for the study of cerebral metabolism, it can be used as an indirect diagnostic tool for brain cancer detection when combined, for instance, with magnetic resonance (MR) brain imaging techniques. In addition to being safe, portable, and affordable, fNIRS lends itself to both non-invasive and invasive applications in combination with other imaging modalities. This chapter provides a review of the application of fNIRS to cancer diagnostics and therapy monitoring. Of particular interest is breast cancer diagnostics where fNIRS has already shown great potential.