Simple SummaryAssistance dogs support humans with a variety of disabilities. Although guide dogs in particular have a long tradition in Western cultures, the legal situation around assistance dogs has been insufficiently developed in many countries so far—a situation that potentially negatively affects both animal and owner. There is also an insufficient amount of research examining assistance dogs in other areas. Studies investigating assistance dogs’ welfare status, cognitive and behavioural capacities, selection criteria for the best fitting individuals, effective training and management practices, and genetic issues, are so far mainly lacking. This review takes a comprehensive approach—it initially outlines important aspects of the current legal situation for assistance dogs in the European Union and Australia, and then it summarizes research findings focusing on dogs’ welfare, cognition, behaviour, health and training. For each of these areas, the need for future development is highlighted and potential ideas for future directions are discussed.Assistance dogs are a very diverse group of working dogs that are trained to assist humans with different types of disabilities in their daily lives. Despite these dogs’ value for humankind, research on their welfare status, cognitive and behavioural capacities, selection criteria for the best fitting individuals, effective training and management practices, and genetic issues are so far lacking. This review highlights the need to address these topics and to promote progress in legal issues around assistance dogs. The topic of assistance dogs is approached comprehensively by outlining the current status of knowledge in three different dimensions: (1) the legal dimension, outlining important legal issues in the EU and Australia; (2) the welfare dimension; and (3) the dimension of research, covering assistance dog selection and training. For each of these three dimensions, we discuss potential approaches that can be implemented in the future in order to support assistance dog working performance, to protect the dogs’ welfare, and to improve our knowledge about them. Additionally, there remain many legal issues, such as the presence of assistance dogs in public areas, the resolution of which would benefit both the assistance dog and the owner with disability.