The nematode species that are of most clinical significance to the horse owner are the large redworms (particularly Strongylus vulgaris) small redworms (Cyathostomins), ascarids (particularly Parascaris equorum) and the pinworm (Oxyuris equi). They have a direct life cycle (no intermediate host) and the females are capable of producing thousands of eggs that can pass out in the faeces and then subsequently contaminate pasture and cause infection in susceptible horses. There are many factors that contribute to the clinical relevance of these species and much research has been undertaken into the factors that cause disease in susceptible equine populations. Many control strategies (for example faecal egg counts) and anthelmintic treatment programmes have been recommended in order to limit the clinical effects of these intestinal parasites; however due to the lack of clinical signs seen in horses most owners have relied on the routine prophylactic use of anthelmintics, and as a result anthelmintic resistance has become a growing concern. A better understanding by veterinary staff of the current research in this area would give them the ability to provide targeted therapeutic advice so that owners have a cost-effective and efficacious means of parasite control.