The interaction between genes and the environment affects all life, both at the level of the individual and on populations. Environment-induced selective pressure can affect changes in gene frequencies resulting in geographic variation in the characteristics of individuals and the emergence of new species. Different organs communicate their status with each other and adapt the individual’s physiology and behaviour to local variations in the environment, for example, after a meal, during stress, or upon exposure to different temperatures. Environmental variation can cause rapid changes in gene expression by modifying regulatory proteins and the expression of non-coding RNA, but it can also cause global and more long-term alterations. The latter include changes to the genome and its packaging, by so-called epigenetic mechanisms. In addition, plastic traits that enable individuals to adapt to expected environmental variation can evolve through natural selection. At Stockholm University, interactions between genes and the environment are studied extensively, including populations adapting to their surroundings and cells responding to environmental change at the mechanistic level. How the environment and genes interact is a central issue for all life on Earth, with clear implications to human health.