What makes the more than 250 cell types in our body remember their identity? What prevents them from becoming cancerous? Why do we inherit some traits from our father, others from our mother? How do our experiences and environment influence our thinking? These crucial questions are all addressed by the field of epigenetics, which studies heritable changes in a phenotype arising in the absence of alterations in the DNA sequence. In this video, Thomas Jenuwein explains the basics of all these epigenetic phenomena characterized by chemical modifications to DNA itself (DNA methylation) or to histones, the proteins around which DNA is wound. These modifications change during development as stem cells give rise to liver cells and neurons, but also in response to environmental signals.