Gene expression states are set by transcriptional activators and repressors and often locked in by cell-heritable chromatin states. Aberrant chromatin states leading to aberrant gene expression patterns (epimutations) can change developmental trajectories and result in disease. Epimutations have been detected in several recognizable syndromes, especially those involving imprinted genes, as well as in cancer. They can result from a DNA mutation in a cis- or trans-acting epigenetic factor (secondary epimutation), or occur as a true or primary epimutation in the absence of any DNA sequence change. Primary epimutations often occur after fertilization and lead to somatic mosaicism. It has been estimated that the rate of primary epimutations is one or two orders of magnitude greater than somatic DNA mutation. Therefore the contribution of epimutations to human disease is probably underestimated.