Psychoneuroimmunology is the science that allows us to understand how several aspects of biology, psychology and sociology are interrelated and involved in achieving successful ageing and consequently healthy longevity. If the ageing process is a biological fact that affects practically all living beings, in social animals such as rats and mice, and especially in humans, social factors can also affect physiological responses, moreover, the psychological characteristics of an individual condition the functional state of the organism. In this context, we have recognize that the maintenance of an appropriate homeostasis, which is carried out through the homeostatic systems, namely the nervous, endocrine and immune systems, is the basis of good health. This healthy state is reached and maintained in different ways for each subject through his/her life and depends on their genes and environmental factors. This fact explains the different rates of ageing, i.e., the biological age, of each individual, and also explains why chronological age does not always coincide with biological age. Because of this the finding of markers of the rate of ageing is very relevant. The use of lifestyle strategies that improve biological age permits the reaching of successful ageing and healthy longevity. In the present chapter, after a brief exposition of the general characteristics of the communication between homeostatic systems, the field of psychoneuroimmunoendocrinology is introduced as the bridge into bio-psycho-social conditions dealing in this Section.