On-farm animal welfare assessment may be predominantly based on resource based measures such as the type of housing, feeding, health plan (e.g. in the Animal Needs Index ) or on animal based measures such as body condition, behaviour or health measures (e.g. in Welfare Quality®). The choice between the two types of measures seems dogmatic, some authors considering that resource based measures detect risk factors for animal welfare whereas animal based measures better reflect the true welfare state of animals. In order to review available resource vs. animal based measures, we first drew a list of properties for a measure to be considered valid for assessing animal welfare. These properties have been derived from analytical methods: selectivity, trueness, reliability (repeatability, reproducibility), stability over time, fitness for the purpose (including sensitivity), and feasibility in different systems. The properties of the measures for the 12 welfare criteria defined in Welfare Quality® were analysed. For some criteria, animal based measures are more suitable than resource based measures, this is the case for absence of prolonged hunger, absence of injuries or diseases or good human-animal-relationship for which different factors interact so that resource based measures have a low predictive value. In other cases, resource based measures are more suitable because animal based measures are not sensitive enough (for absence of thirst) or not feasible (detection of pain during some management procedures on farm). Finally, for some criteria a combination of resource and animal based measures is necessary. This is the case for comfort around resting where problems (negative aspects) can be detected on animals while positive aspects – the actual comfort – may be assessed through resource based measures. In conclusion, there is no general rule whether animal based measures are more or less valid than resource based ones. This depends on the welfare criterion considered and to a lesser extent on the animal type. We recommend to use a mixture of animal and resource based measures (chosen according to their validity and feasibility) to assess the overall welfare of animals on farms.