In the environment, any organism is confronted with stress, for instance, stress caused by fluctuating or changing environmental conditions and disturbances by man or by pollution. Such stress may be harmful to biological life, exhibiting adverse effects at different levels of organization, such as populations, individual organisms, tissues, and cells. These effects are demonstrated in life-history traits of an organism, for example, development, growth, aging, longevity, survival, and reproduction. Biologists borrowed the term stress from physics, where it indicates force per unit area, so in this case stress is caused by an external agent acting on a system. In biology, however, stress became synonymous with the internal consequences of an external factor. There seems to be agreement now that a distinction should be made between stressor (an external factor), stress (an internal state brought about by a stressor), and the stress response (a cascade of internal changes triggered by stress). Although the concept of stress can be defined at various levels of biological integration, varying from cells, individuals, populations, and ecosystems (Parker et al., 1999), stress is most commonly studied in the context of individual organisms (Maltby, 1999), and the responses on the level of cells and biochemical pathways (as covered in this book).