Healthcare in the 21st century has moved from a disease-centred perspective to a patient-centred one, in which the concept of quality of life plays a crucial role. Nowadays, with an ageing population throughout most of the world and an increased life expectancy, there is a large number of individuals living with physical and/or mental chronic illnesses/disabilities. Therefore, the focus of medicine has shifted towards the quality of survival, and not only on the mere length of life. For these reasons health - related quality of life (HRQoL) has become an important field of study in medical care. The present work focuses on the definitions and domains regarding HRQoL. But defining what quality of life is involves also ethical considerations: when decisions about severely ill or disabled patients need to be taken, having a clear idea of what a good/bad quality of life is becomes essential. Clinicians and researchers in the health care should be aware of the importance of HRQoL and its implications. Indeed the concept of HRQoL allows professional caregivers to understand the patient’s evaluations and perceptions of his/her illness/disability and of the related treatments. Moreover it allows comparisons among different interventions and their respective effectiveness.