Eukaryotic genomes are divided into chromosomes, each consisting of a single molecule of several centimeters of DNA compacted into a nucleoprotein substance known as “chromatin”. In the recent years, evidence has accumulated pointing out chromatin polymorphism and dynamics as a primary mean of control of genome accessibility in time and space, driving the focus on this complex polymer as a critical player in gene regulation [1]. A thorough characterization of chromatin properties would then be a prerequisite step in our understanding of differential gene expression, e.g. “epigenetics” in its original definition by Waddington as “the study of the causal mechanisms by which the genes of the genotypes bring about phenotypic effects”.We wish here to emphasize some physical characteristics of genome organization in order to provide a more complete framework in which to interpret the control of gene expression. Indeed, cells achieve stability and heritability of epigenetic states by taking advantage of many different physical principles, such as the universal behavior of polymers and copolymers, the general features of non-equilibrium dynamical systems, and the electrostatic and mechanical properties related to chemical modifications of DNA and histones [2]. At a shorter time scale, as various molecular motors push, pull and twist DNA, transient forces and torques develop within chromatin, with expected consequences on transcription [3]. This new perspective allows rationalizing the normal cellular functions.[1] Boule JB, Mozziconacci J and Lavelle C. (2015). The polymorphism of the chromatin fiber. J Phys Cond Mat 27(3):033101.[2] Cortini, R, Barbi M, Care B, Lavelle C, Lesne A, Mozziconacci J and Victor JM. (2015). The physics of epigenetics. Rev. Mod. Phys. (in press).[3] Lavelle C. (2014). Pack, unpack, bend, twist, pull, push: the physical side of gene expression. Curr Opin Genet Dev 25:74-84.