Yeasts are eukaryotic, unicellular microorganisms widely distributed in the natural environment. Their properties make them extremely valuable organisms in many industrial processes, including biotechnology, pharmaceuticals, and food. In the environment and biotechnological processes, yeasts are exposed to various stress factors, such as temperature or pH changes, osmotic pressure, nutrient deficiencies, high ethanol concentrations, toxic compounds, and factors that generate oxidative stress. As a result of their action, several physiological and morphological changes occur, which are the leading cause of the decrease in viability and vitality of yeast cells under stress conditions. Most biotechnological applications of yeast involve the species Saccharomyces cerevisiae, which explains why it is predominantly used as a research model to analyse cellular responses to stress or the mechanisms of adaptation to external environmental changes. This knowledge is important for understanding the biological basis of cell responses to stress and for ensuring the appropriate efficiency of biotechnological processes. This chapter focuses on oxidative stress in the context of the factors that induce it, its consequences for the cell, and the mechanisms associated with tolerance to stressful conditions.