All cells of the body, regardless of the tissue type, contain the same genetic material, but express this genetic material differently. Epigenetics is one process by which differential gene expression within a cell is regulated. Epigenetic mechanisms involve postsynthetic modifications to DNA and/or DNA-associated histones that do not change the DNA sequence itself, but which remodel chromatin, are passed along at each cell division, and occur during and after early development. The CD4+ T cell best represents a cell in which epigenetic mechanisms are used to affect mature cell physiology. As a naïve CD4+ T cell develops into either a Th1 or Th2 cell that secretes predominantly IFN-gamma or IL-4, respectively, the expression of one cytokine gene and the permanent silencing of the other is orchestrated using epigenetic mechanisms. Because there appears to be an association between Th1/Th2 cell immunity, behavior, and/or disease, it is possible that an environmentally induced epigenetic change that occurs during Th1/Th2 cell development could explain how certain Th1/Th2-associated conditions develop. This article will review basic epigenetic mechanisms and what is known about how these mechanisms influence cytokine gene expression in a naïve CD4+ T cell as it develops into a Th1 or Th2 cell.