Currently, six basic allergic eye diseases are recognized. In seasonal (SAC) and perennial allergic conjunctivitis (PAC), the allergic response is mediated predominantly by mast cells, whereas the more severe conditions, vernal (VKC) and atopic keratoconjunctivitis (AKC) and giant papillary conjunctivitis (GPC), are associated with a preponderance of T cells. Acute allergic conjunctivitis (AAC) occurs when a large quantity of allergen inoculates the eye and is usually self-limiting. SAC, the most common ocular allergy, is the ocular component of hayfever. PAC in the UK is most commonly caused by the house-dust mite (HDM); diagnosis is confirmed by skin-prick tests, eosinophils in the conjunctival smear, and raised tear or serum total IgE. SAC and PAC can usually be managed with chromone eyedrops and antihistamines. VKC usually presents in children under 10 years of age and mainly affects boys. Sufferers frequently have a personal or family history of atopy. Corneal involvement can occur in VKC, making it potentially sight-threatening. AKC occurs in atopic adults, and like VKC it affects the cornea. VKC and AKC require steroid treatment under specialist supervision; minimization of the steroid dose can often be achieved with use of a chromone. GPC occurs due to repeated contact of the conjunctival surface with a foreign surface, such as contact lenses. Attention to lens hygiene or switching to different lenses and treatment with a chromone are frequently effective. In all allergic eye diseases contact with the precipitating allergen should be avoided as far as possible.