In this paper we present methods that we have developed to measure pigmentation in human skin. This involves the measurement of diffuse reflection spectra from human skin in vivo and referencing them to either totally depigmented skin, or the skin of the same individual if we are measuring variations in pigmentation. Changes in the pigmentary system brought about by UV radiation can be measured for each individual. Absolute measurements lead to estimates of the melanin concentration in the skin, while differential measurements lead to estimates of the quantity of additional or reduced pigment content of some skin lesions. The same instrumentation has been successfully used to assess UV-induced erythema as well as other vascular changes. The determination of the minimum detectable erythema dose can be performed even in the darkest-skinned subjects without loss of sensitivity as in the case of laser Doppler instruments. It has been shown that what is perceived by the eye as erythema is a very complex phenomenon, encompassing a large number of vascular reactions that can be studied in detail through diffuse reflection spectroscopy. Some of the possible responses are presented, as well as the contributing chromophores that have been identified so far.