Fleas are the most important ectoparasites of dogs and cats worldwide. The annual cost to control fleas in companion animals exceeds $1 billion in the USA and €1.1 billion in Western Europe. As well as acting as vectors of disease, and a source of owner revulsion and bite reactions, they are also the most common causes of allergic dermatitis in cats and dogs. Similarly, biting flies are a major source of allergic skin disease in horses. An allergy is an exacerbated response from an individual when it comes into contact with foreign substances (allergens) such as flea and fly saliva. This article considers mechanisms of flea and fly bite sensitivity, diagnosis, and the role of the veterinary nurse in the prevention of these diseases.