Xeroderma pigmentosum (XP) is a rare inherited, heterogeneous syndrome with pigment anomalies, sun sensitivity, multiple cutaneous neoplasms and abnormal self protecting systems (SPS). The transmittence is autosomal-recessive. 50 percent of XP patients gets melanoma and 15 percent have neurological abnormalities. Clinical differentiation, determination of the DNA repair rate and cell fusion studies allow the differentiation of 6 complementation groups including De Sanctis-Cacchione syndrome and the XP variant typ. Pigmented Xerodermoid is a special form. Cytogenetic studies give evidences for the model character of XP for UV carcinogenesis.