In biology, the term mosaic means an organism that is composed of two or more genetically different cell lines originating from one homogeneous zygote. Mosaicism can occur in all pluricellular living organisms. Molecular genetics has provided evidence that every human being represents, to some degree, a mosaic. In dermatology, the concept of mosaicism has helped solving the following problems: How to find a workable definition of the term nevus; why different types of nevi may sometimes occur as twin spots; why the arrangement of nevi can neither be called “dermatomal” nor “zosteriform”; why we can discriminate, in autosomal dominant skin disorders, three different categories of postzygotic mosaicism; why some segmentally arranged skin disorders are heritable whereas others are not; why a segmental distribution of autosomal dominant skin disorders implies an increased risk for the next generation; why patients born with a severe Mendelian skin disorder may develop, later in life, patchy areas of healthy skin; why common skin disorders such as atopic dermatitis or lichen planus are sometimes superimposed by a rather pronounced linear or otherwise segmental involvement; and why all types of skin cancer reflect mosaicsm.