Simple Summary: Horses are used for a wide range of different purposes and may be vulnerable to a large number of different welfare issues, some of which are unique to equines, such as responses to being ridden or trained. Compared to farmed livestock, their welfare has received less attention, but concern for their welfare is increasing. Welfare issues can arise from the environments in which animals are kept, how they are treated by their human caregivers and their health. To determine which of the issues are most important and may need the greatest attention in terms of research effort or owner education, we used a process of eliciting expert opinion. Through a series of surveys and ranking of issues, we determined that, in the opinion of equine experts, the most important issues for horses were poor disease prevention, issues arising when old or sick horses are not promptly euthanized, lack of owner knowledge of welfare needs of horses, fear and stress involved in horse use, inability of owners to recognize pain behaviour, obesity and inadequate feeding practices. Prioritizing different welfare issues can help to focus attention on the most pressing or severe issues causing the greatest amount of suffering.Abstract: Equine welfare issues are receiving increasing attention in the UK, but welfare problems can arise from a wide range of causes. In order to identify the most important welfare concerns for horses, we used a Delphi method with 19 equine welfare experts. An initial list of 84 equine welfare issues was generated using an online discussion board and NVivo thematic analysis. Subsequently, experts ranked these welfare issues for perceived prevalence, severity and duration of suffering associated with each issue on a 6-point Likert scale. All issues with a mean score of 3 or above (n = 37) were included in subsequent rounds. Finally, a subset of experts attended a two-day workshop to determine the final priority list of welfare issues. The welfare issues perceived to be most prevalent were lack of biosecurity, delayed euthanasia, lack of owner knowledge of equine welfare needs, fear and stress from use, and obesity. The issues considered to cause greatest suffering for individual horses were delayed euthanasia, lack of recognition by owners of pain behaviour, large worm burdens, obesity and being fed unsuitable diets for equine feeding behaviour. These outcomes can help to focus research and education interventions on the most pressing welfare issues for horses.