For several centuries, optical methods have provided powerful tools to measure an assortment of physiological variables. However, the use of diffuse optical light for physiological measurements and, in particular, for noninvasive monitoring, has a somewhat shorter history. Unlike traditional ray optics as used in light microscopes, the eld of diffuse optics pertains to the propagation of light through highly scattering samples, such as biological tissue. In this diffuse regime, the photons can no longer be assumed to travel in a ballistic-like path through the sample, and instead the equations of light propagation must be replaced by stochastic models to account for multiple scattering events and the numerous direction changes experienced by these photons as they migrate through scattering mediums. On one hand, diffuse optical techniques allow measurements through much thicker samples than are possible with traditional optics, including the ability to record noninvasive measurements from the human brain, which is the subject of this chapter. However, on the other hand, there are penalties to be paid for this ability and the progress of these techniques has faced many technical and methodological challenges.