Histiocytoses are a heterogeneous group of disorders that are characterized by the proliferation and accumulation of reactive or neoplastic histiocytes. Three classes of histiocytoses have been defined: class I, Langerhans cell disease; class II, non-Langerhans cell histiocytic disease without features of malignancy; and class III, malignant histiocytic disorders. Although the disorders in classes I and II usually have a benign appearance on histology and are commonly non-aggressive and self-healing, some can cause debilitating or even fatal outcomes. Such cases beg the question: what stimulates aggressive behavior of a classically benign disease? New molecular information may now provide insight into the driving force behind many of the aggressive histiocytoses. In this article, we review Langerhans cell disease and seven aggressive histiocytoses that can involve skin, discuss histologic features that may forecast a poor prognosis, and discuss the molecular findings that help to explain the pathophysiology of these aggressive histiocytic disorders.