Animal welfare is multidimensional; its assessment relies on complementary measures covering all dimensions. Welfare Quality® constructed a multicriteria evaluation model for its assessment at unit level (farms, slaughterhouses). Four welfare principles are distinguished (‘Good feeding’, ‘Good housing’, ‘Good health’, and ‘Appropriate behaviour’). An animal unit receives four principle scores (expressed on a 0-100 value scale). These scores are aggregated together to form the overall assessment by sorting animal units into predefined welfare categories boundaried by reference profiles. A unit is assigned to the welfare category above the profile it is considered at least as good as. Several assignment procedures were tested on a set of 69 dairy farms and compared with observers’ general impressions. The welfare categories, reference profiles and assignment procedure were defined in consultation with social scientists, animal scientists and stakeholders. Four welfare categories were defined: ‘Excellent’, ‘Enhanced’, ‘Acceptable’, and ‘Not classified’. The reference profiles were set at 80, 55 and 20, corresponding to aspiration values for Excellent, Enhanced and Acceptable. The assignment procedure resulted from a compromise between theoretical opinion on what should be considered excellent, enhanced or acceptable, and what can realistically be achieved in practice: to be assigned to a given category, a unit must reach its aspiration value on 2 or 3 of the 4 principles, and not score below the aspiration value for the next lowest category on the other principle(s). The model can be used for several purposes, including identifying welfare problems on a farm to advise farmers, or checking compliance with certification schemes.