Effective parasite control is essential for the maintenance of optimal health and performance in the horse. The worldwide escalation of parasite resistance is a major cause of concern for the horse industry. Parasite resistance to every main class of equine anthelmintic has been documented. Furthermore, dual and cross resistance also have been widely reported, despite different climatic and management practices seen throughout the world. Studies documenting parasite resistance to major classes of equine anthelmintics are discussed. Disagreement among researchers exists regarding how to effectively control equine internal parasites. Current theories of factors leading to resistance and control programs are discussed. It is clear that parasite resistance and control in the horse is an area requiring continued intensive study.