In this chapter the systems that form the basis of our bodily processes (e.g. metabolism, digestion, circulation, excretion, respiration and motor activities) are described, as well as the mechanisms that regulate those processes and keep our body in balance: protection and defence by our immune system, hormonal regulation by our endocrine system, perception and behaviour by our nervous system, and last but not least the overall integration and control by the central nervous system. External stressors can influence all three control systems of the human body and can result in malfunctioning of the bodily processes, presented in both mental and physical effects. With that in mind this chapter provides the background knowledge required to understand the stress mechanisms that can lead to those effects.