CD4(+) helper T type 1 (Th1) and Th2 cells are critical mediators of inflammatory diseases. Although T cells represent only a fraction of the leukocytes that are found in the lung during inflammation, they play a critical role in coordinating the immune response to infectious agents and allergens. T cells have the ability to rapidly expand in response to specific stimuli and to differentiate into effector cells that, through the production of soluble factors such as cytokines and chemokines, communicate with other cells to initiate a cascade of inflammatory events. The objective of this review is to outline the cellular and molecular mechanisms involved in the generation and recruitment of Th1 and Th2 cells in the lung. Defining these mechanisms should lead to improved immunopharmacological strategies for prophylaxis and therapy.