Abstract: Exponential advances in the quantitation of DNA variation and epigenetic states seem poised to convert much of biological research into a statistical exercise. But these developments also invite us to reimagine well-worn biological concepts on a grander scale. Somatic mosaicism refers to postzygotic mutations persisting in the individual, occasionally conspicuous to dermatologists as Blaschkoid, checkerboard, phylloid and patchy morphologies. A thoughtful examination of cutaneous mosaicism suggests, however, that virtually all of us may be somatic mosaics. Such genetic variability within individuals might explain localized presentations of disease and implies that some tissues literally evolve throughout life. We discuss here (i) the likely ubiquity of somatic mosaicism, (ii) the broad range of possible biological consequences and (iii) how experimentalists and clinicians may begin establishing genotype-to-phenotype correlates.