Although the term 'quality of life' (QoL) is not unfamiliar to veterinary surgeons, only recently has the scientific community attempted to measure it in farm and companion animals. Typically such studies have applied methodologies from the field of human health-related quality of life
(HRQoL), without due consideration of the applicability of both the term and its measurement to animals. However, it is necessary to clarify the philosophical basis of QoL if it is to be defended as a rigorous and reliable aid to decision-making in animal welfare science. In this paper we
review common concepts in human HRQoL and discuss the value of, and difficulties regarding, the transfer of the concept of human HRQoL to companion animals. Human definitions tend to focus on individuals and their assessment of the state of their life in terms of physical, social and psychological
functioning. The use of the term 'quality of life' for animals may therefore expand on what is usually considered when using the term 'welfare', and thereby improve on current practice, which tends to focus on relatively few outcome measures that are largely indicative of poor welfare. However,
failure in the human literature to properly define QoL and defend the choice of measures accordingly, together with the common use of objective indicators and proxies, has led to confusion over the relative roles of objective and subjective measures in the determination and constitution of
QoL. A suggestion for an appropriate definition of animal QoL that clarifies these relationships is offered, together with a list of social/environmental and physical/psychological health-related domains that may be suitable for a generic companion animal QoL assessment tool. In the absence
of knowledge on both basic needs and individual preferences, particularly for institutionalised animals, QoL tools may be more appropriately designed as outcome-based tools, focussing on observable signs of health and behaviour. The extent to which recent QoL assessment tools for companion
animals have covered these domains, and the extent to which the psychometric properties of the tools have been addressed, is also briefly discussed.