Simple SummaryZoos and aquariums have recognized the paramount importance of animal welfare. Achieving and maintaining the highest possible standards of animal welfare is not only a moral obligation but also a necessary condition for zoos and aquariums to fulfil their educational and conservational functions. However, evaluating the welfare of zoo animals presents a challenge due to the wide range of species involved and a lack of knowledge regarding the specific requirements of each species. Therefore, to protect and promote the welfare of zoo animals, there is a need for science-based tools to assess and monitor it. This review article aims to discuss the advantages and limitations of existing methodologies for assessing animal welfare in zoos, describe the main animal-based welfare indicators for zoo animals, and identify areas that require further research.AbstractZoological institutions, such as zoos and aquariums, have made animal welfare a top priority, as it is not only a moral obligation but also crucial for fulfilling their roles in education and conservation. There is a need for science-based tools to assess and monitor animal welfare in these settings. However, assessing the welfare of zoo animals is challenging due to the diversity of species and lack of knowledge on their specific needs. This review aims to discuss the advantages and disadvantages of existing methodologies for assessing zoo animal welfare through: (1) A critical analysis of the main approaches to zoo animal welfare assessment; (2) A description of the most relevant animal-based welfare indicators for zoo animals with a particular focus on behavioural and physiological indicators; and (3) An identification of areas that require further research.