Atopy comes from the Greek word atopos, meaning strange or unusual. In 1892, Besnier was the first to describe the association of AD with allergic rhinitis and asthma^[1]. Decades later, the term ‘atopy’ was coined by Perry, as quoted by Coca & Cooke^[2], to describe this triad of atopic eczema, allergic rhinitis and asthma. In the 1930s, Hill & Sulzberger^[3] named the cutaneous aspects of the triad ‘atopic dermatitis’, replacing the terms disseminated neurodermatitis and atopic eczema. It was not until Hanifin & Rajka^[4] proposed a list of characteristic features (‘criteria’) in the 1980s that unity in the clinical concept of AD was established (Table 13.1). In 1994, the UK Working Party refined Hanifin & Rajka's criteria into a concise and validated set of survey-based diagnostic criteria useful for the purposes of epidemiologic studies (Table 13.2)^[5–7].