The concept of epigenetics refers nowadays to the long-lasting and inheritable gene expression states that are established in the absence of a change in the DNA sequence itself. The fast progress of the field in later times has provided scientists with novel understanding on how the environment in the wide sense, including nutrition, exercise, even behavior of the organisms, participates in the regulation of gene expression. The molecular mechanisms that mediate epigenetic regulation are principally DNA methylation, the post-translational modifications of the histones and the regulation by non-coding RNAs. They are intimately related to cell differentiation and developmental plasticity and relay environmental influences to the cell nucleus, thus bridging the gap between lifestyle and the genome. In this introductory chapter we offer a general vision of the well-documented effects of lifestyle choices and environmental impact on gene expression and present the reader with a general overview of the better known epigenetic mechanisms and the techniques most often used to study them.