To qualify and quantify animal welfare, novel assessment tools have been and are being developed, while existing assessment tools are being modified so that they can be applied to multiple species living under different housing and management conditions. The results of such assessments should be reliable, consistent and reproducible. We review the steps that should ideally be taken to develop, validate and apply animal welfare assessment tools. The first step should be to find a definition of animal welfare that the various stakeholders can agree upon. The second step should be to formulate and agree upon a framework for the evaluation of animal welfare. Both theoretical/conceptual frameworks, which provide a structure for research and suggest which facets are considered important, and ethical frameworks, which explicate the underlying moral position, should be considered. Finally, animal welfare assessment tools should be developed and validated based on both the adopted welfare definition and the welfare evaluation framework(s). However, this three-step approach has not always been followed in the development of welfare assessment tools currently in use. We expect that transparency and clarity regarding the underlying definitions and frameworks will increase the likelihood that the resulting welfare assessment tools will give similar weight to the aspects considered relevant to animal welfare, as it helps to specify the aspects that are considered to be key elements of animal welfare. This approach should lead to convergent assessment results and higher correlation of welfare indicators between assessment tools.
