This course explains principles and applications of diffuse optics in biology and medicine. Diffuse optical tools have a large and rapidly expanding set of pre-clinical and clinical applications. We will focus on measurements of ‘thick’ tissues (<1cm) with diffuse optical spectroscopy, diffuse correlation spectroscopy, diffuse optical tomography, and diffuse fluorescence. The primary goal of this course is to provide attendees with sufficient knowledge to grasp the underlying concepts, strengths, and weaknesses of the technologies involved. Examples will be taken from preclinical and clinical applications, especially critical care. Researchers considering use of diffuse optical tools, clinicians in search of technologies to address pressing monitoring needs, and translational researchers interested in applying diffuse optics will benefit from the course.