Yeasts are ubiquitous microorganisms that form part of the microbiota of most if not all natural ecosystems. A wide variety of yeast species occur in soil, fresh water and marine water, they are normal inhabitants on plants, are commonly associated with animals, and are also found frequently in man-made habitats such as foods. The conditions prevailing in these natural and artificial habitats determine the metabolic activity, growth and survival of yeasts. A variety of abiotic and biotic factors influence the life of yeasts, and exert stress conditions which the cells must withstand and adapt to or otherwise they die. Death of individual cells may not bring about the extinction of a whole population as there is a large variation in resistance to stress factors among cells. Also, these factors change with time and show spatial heterogeneity; their effect is manifested differently in microenvironments. So, when looking at the environmental relations of yeasts, we should deal with a very complex interrelation of factors that are only partially understood. Nevertheless, basic knowledge of these is important for understanding the ecology and biodiversity of yeasts as well as to control the environmental factors in order to enhance the exploitation of yeasts or to inhibit or stop their harmful and deleterious activity.In this chapter our current knowledge of the environmental relations of yeasts is summarized. First, the most relevant physical, chemical and biotic factors and their interactions are described in a changing environment. Further on, responses of yeast cells to stress conditions are discussed at single cell and population level. In-depth treatment of the genetic background of variation, and the mechanism of adaptation to stress factors would be beyond the limits of this chapter. Further chapters in this book are devoted to their discussion. Reference is made to relevant literature and to the large amount of data covered and summarized in previous works and reviews