On one hand, the advancement of animal welfare science seems to be a slow process; on the other hand, quick solutions to animal welfare issues tend to result when legislative bodies become involved. The ‘quick fix’ approach to animal welfare can be dangerous. It can result in research that is too narrow, where scientists do not consider the whole picture. The difficulty in defining animal welfare has led to many definitions and concepts being developed. Some definitions have removed important components that are difficult to measure, such as feelings, emotions and consciousness. These definitions tend to focus on animal health and biological functioning as measures of animal welfare. Animal welfare becomes easier for scientists to measure, but the results do not address the concerns of the general public, which tend to be focused on feelings and emotions. Currently, validation of welfare indicators is lacking. If the focus is on biological functioning, must show that this is related to suffering. There is also a lack of using health problems as indicators of welfare, which are easily validated. For example, lameness is obviously linked to pain, so welfare could be compared in different housing systems based on this measure. Different housing systems are difficult to assess because there are so many variables that could challenge animal welfare. Two approaches for assessment include: General welfare indicators, or an overall assessment, such as longevity in the system. Another approach is to document the incidence of all the problems in each system and then compare them. However, it is difficult to integrate all the measures of welfare. A new housing system may solve welfare problems, but create others in another area. Therefore, it is difficult to develop an overall welfare score, when measures point in different directions. This problem raises the question of possibly weighting the indicators. However, this must be done objectively either statistically or based on a theoretical model. The focus may turn towards using critical measures or solving the problems associated with each housing system, as opposed to attempting to assess overall welfare. There needs to be more focus on the variation that exists within a particular housing system (usually due to different management factors), not just between systems. There also needs to be a better understanding of the biological basis of behavioural, physiological and immunological responses that are used as indicators of welfare. A single definition or concept of welfare may never be achieved; but that should not deter the effort to improve it for animals. Perhaps, instead of a single concept, multiple concepts need to be considered.