Many biomedical applications of lasers have been developed. In many cases, however, selection of a particular laser has been based largely on availability, and treatment protocols have often been developed by an empirical approach. The choice of lasers and exposure parameters must be based on an understanding of the mechanisms of treatment, the optical and thermal properties of the tissue, or rational, theoretical approach to maximize therapeutic benefits and minimize adverse reaction such as scarring. Selective photothermolysis has been proposed as a construct which may be useful in designing laser systems for biomedical applications, especially for the treatment of pigmentary dermatoses. The necessary conditions are (1) tissue structures which preferentially absorb a wavelength of light more than the surrounding tissue,(2) a pulse of light sufficiently short and energetic to produce localized high temperatures in the absorbing structures, and (3) a predominantly thermal mode of initiation of tissue damage. The concept of selective photothermolysis has been confirmed by our gross and ultrastructural observations of albino and black guinea pig skin irradiated with single laser pulses. At present, various kinds of Q-switched lasers, with a pulse of light sufficiently short and energetic enough to produce localized high temperatures in the targets, are used for the treatment of many pigmented skin lesions and demonstrated to be useful for treatment of dermal melanocytosis and black, brown, and blue tattoos which showed no significant response to previous therapies. In the patients with basal pigmentation, however, patients with postinflammatory pigmentation or chloasma show no response, although most patients with solar lentigo and all patients with mucosal pigmentation reveal a good response after only one laser treatment. Therefore, it becomes more important to differentiate several pigmented lesions responsive for laser treatment from the others.