This chapter contains sections titled:

• Introduction

• Definition of atopy and IgE sensitization

• Definitions of Asthma, Hay Fever, and Eczema

• IgE sensitization and asthma and allergic disease

• Geographic variation

• Time trends

• Personal risk factors

• The “hygiene hypothesis,” infections, farming, and parasites

• Genetics

• Exposure to allergen

• Diet

• Smoking

• Air pollution

• References