Allergies, also known as allergic diseases, are a number of conditions caused by hypersensitivity of the immune system to typically harmless substances in the environment. These diseases include hay fever, food allergies, atopic dermatitis, allergic asthma, and anaphylaxis.Allergic diseases are caused by allergen-induced unfavorable immune responses initiating various symptoms in different organs, which often cannot be completely controlled by modern medicine. The immunologic basis of allergic diseases is observed in two phases: sensitization and development of memory T and B cell responses, and IgE production and effector functions, which are related to eosinophils, innate lymphoid cells, dendritic cell subsets, epithelial cells and tissue inflammation/injury, epithelial barrier, tissue remodeling and chronicity in asthma, atopic dermatitis (AD) and allergic rhinitis (AR). Different disease phenotypes and endotypes may become apparent with different dominant molecular mechanisms, related biomarkers and responses to biologic therapy. Multiple mechanistic factors are complexly involved in the pathogenesis of allergic inflammations, therefore, comprehensive understanding of their mechanisms is required to develop efficient ways of prevention and further advance treatment modalities. In this chapter, we discuss cellular and molecular mechanisms of allergic diseases.