Caterpillars are the larval forms of moths and butterflies and belong to the order Lepidoptera. Caterpillars, and occasionally moths, have evolved defense mechanisms, including irritating hairs, spines, venoms, and toxins that may cause human disease. The pathologic mechanisms underlying reactions to Lepidoptera are poorly understood. Lepidoptera are uncommonly recognized causes of localized stings, eczematous or papular dermatitis, and urticaria. Part I of this two-part series on caterpillars and moths reviews Lepidopteran life cycles, terminology, and the epidemiology of caterpillar and moth envenomation. It also reviews the known pathomechanisms of disease caused by Lepidopteran exposures and how they relate to diagnosis and management. Part II discusses the specific clinical patterns caused by Lepidopteran exposures, with particular emphasis on groups of caterpillars and moths that cause a similar pattern of disease. It also discusses current therapeutic options regarding each pattern of disease.