The prevalence of childhood allergic diseases, such as allergic asthma, allergic rhinitis, and atopic dermatitis, has increased exponentially. In Singapore, the prevalence of asthma at all ages exceeds 20%, and around 50% of Singaporean children show features of an underlying allergy. The exact environmental causes for the increase of allergic diseases have not yet been identified, but most researchers agree that a decreased bacterial load in young children may be one of the reasons for the increase. However, the causes of allergy are multiple, and the development of an allergic disease is the result of complex interactions between genetic constitution and environmental factors. In this review article, different aspects of allergic sensitization are covered, including prenatal and postnatal sensitization. The phenomenon of the "allergic march" (switching from one clinical expression of allergy to another) and its underlying mechanisms are discussed. The last part of this review article is on prevention and treatment of allergic diseases, including the role of bacterial products (probiotics, prebiotics, and synbiotics) and the role of immunotherapy, including sublingual immunotherapy.