The discovery and use of new optical windows where short-wavelength infrared (SWIR) light can be transmitted between areas of intense water absorption is a major development in photonics. This book covers biomedical uses of light at the conventionally used first and second optical windows, and, in particular, explores emerging applications of SWIR light at a third and a fourth optical window (at 1600–1870 nm and 2100–2350 nm, respectively) in the SWIR range. Diagnostic techniques that utilize these windows are reviewed, as are applications to cancer and diseases of organs such as the heart, brain, and skin. The book ends with an extensive discussion of the potential uses of artificial intelligence to enhance the ability to study these diseases at SWIR optical windows.