This chapter introduces fundamental principles of public health and their applications to the study and understanding of disasters and medical humanitarian assistance. It also covers basic terminology definitions and theories of public health. Public health has been defined as 'the science and art of preventing disease, prolonging life and promoting health through organised efforts of society'. In the context of disaster preparedness, response and management, the definition of health implies the importance of addressing all three fundamental components of health: the physical, social and mental well-being of a population. Public health is a multidisciplinary field in medicine that utilises epidemiology, clinical trials, biostatistics, laws and ethics to protect health, improve health and secure the provision of health services. The socio-ecological approach in public health connotes health is governed by many external factors, not just individual biological make-up or lifestyle choices. Primary prevention is concerned with measures that prevent the onset of disease.