Welfare is a multidimensional construct and its quantification is of major scientific, societal and economic importance in veterinary medicine. The construction of indices that measure welfare validly and reliably remains a considerable challenge. A general methodology for constructing welfare indices can be adapted from human medicine (in particular, from methodologies to assess Quality of Life [QoL]) and modified to reflect the fact that all assessments of animal welfare must be observer-based. The methodology is based on the creation of individual, composite indices for distinct dimensions/domains of welfare such as pain, disease, or stress. The domains include behavioural, physiological and biochemical markers. We have established QoL methodologies in the assessment of acute and chronic pain in dogs and generalised this approach to farm animal welfare. We describe the development of a questionnaire with seven behavioural categories which are used to create a single pain score to assess acute pain in dogs. For chronic pain in dogs, a structured questionnaire with over 100 items has been devised, which the owner completes by indicating degree of agreement with each item using a seven-point Likert scale. The welfare measure includes pain as an integral component as well as husbandry, behavioural and physiological/biological measures. In each case, a profile of the individual indices can be studied and compared over time or among observers. These indices may also be combined to form a single composite welfare measure, should this be appropriate, using scaling models. In the welfare setting, we have both causal and indicator variables — and indeed, for farm animals, the causal variables may be sufficient cause for poor welfare (eg the presence of disease or inadequate husbandry).