This book is a consideration of the branch of heredity known as EPIGENETICS and its implications for a variety of diseases in humans and animals. It is written to be accessible to a general audience, assuming a minimal background in biology. The first chapters presents historical background detailing the growth in our understanding of the science of genetics and how it grew and expanded during the 20th century. It includes some discussions of the molecular mechanisms of the epigenetic control of gene expression, but its main focus is the gathering body of evidence connecting it to a range of the most significant illnesses, including cancer, autism, Alzheimer’s disease, diabetes, and other conditions. The writing stresses areas of uncertainty, as well as scientific debates concerning the role of environmental factors in the increasing frequency of many of the conditions under consideration. The final chapters discuss the implications for society and lay out options for meeting the challenges of the coming years. While the work is designed to be an easy and straightforward read, there are included extensive endnotes in which some ancillary details and personal anecdotes are made available to readers who wish to pursue related questions in modern biology. The work is not intended to be a scientific document aimed at professionals in the field; there are already many books and reviews on these topics, at all levels of scientific detail and complexity. Rather, the bottom line is to add to the public discourse on the grave issues that the new science of epigenetics is bringing to the fore.