Health psychology is the study of psychological and behavioural processes in health, illness, and healthcare. It is concerned with understanding how psychological, behavioural, and cultural factors contribute to physical health and illness. Psychological factors can affect health directly. For example, chronically occurring environmental stressors affecting the hypothalamic-pituitary–adrenal axis, cumulatively, can harm health. Behavioural factors can also affect a person's health. For example, certain behaviours can, over time, harm (smoking or consuming excessive amounts of alcohol) or enhance health (engaging in exercise). Health psychologists take a biopsychosocial approach. In other words, health psychologists understand health to be the product not only of biological processes (e.g., a virus, tumour, etc.) but also of psychological (e.g., thoughts and beliefs), behavioural (e.g., habits), and social processes (e.g., socioeconomic status and ethnicity).

By understanding psychological factors that influence health, and constructively applying that knowledge, health psychologists can improve health by working directly with individual patients or indirectly in large-scale public health programs. In addition, health psychologists can help train other healthcare professionals (e.g., physicians and nurses) to apply the knowledge the discipline has generated, when treating patients. Health psychologists work in a variety of settings: alongside other medical professionals in hospitals and clinics, in public health departments working on large-scale behaviour change and health promotion programs, and in universities and medical schools where they teach and conduct research.