In this chapter light is principally described as particles with energy hν and velocity c. These particles are scattered or absorbed by structures in turbid media such as biological tissues and are reflected at boundaries between media with different refractive index, according to the laws of Fresnel. We will only consider monochromatic light, which covers most practical situations in medical and biological laser applications. The formulations are easily extended to polychromatic light as long as scattering effects are elastic. The theory becomes more complicated for inelastic scattering, like in fluorescence. However, even then the diffusion theory is a straightforward extension of the discussions presented in the following sections. Finally, we neglect polarization and interference. To include polarization one would need four diffusion equations instead of one [1].