In the past decade, there has been an explosion of interest in the connective tissue of the body. The authors know now that the 3 general categories of connective tissue, collagen, elastic fibers, and proteoglycans, are all heterogeneous, with multiple types and substructures. In addition, numerous enzyme systems have been discovered that are critical for preparing each connective tissue component for its structural function. Connective tissue is vital to the structure and function of every tissue; it gives blood vessels viscoelastic properties; skin and tendon, thoughness; bone, a matrix to calcify. The lung, of course, is no exception. Connective tissue comprises approximately 25% of the adult human lung and is an intimate part of all lung structures. The lung is the one organ of the body in which we can estimate the contribution of connective tissue to mechanical behavior in vivo and in which abnormalities in connective tissue very quickly appear as abnormalities in function. The purpose of this review is multifold: to define the components of connective tissue in general terms and, specifically, as related to lung; to describe the mechanisms controlling the presence of connective tissue in the lung; to summarize the current concepts of how connective tissue influences lung structure and function in both health and disease; and to discuss some possible future directions for the study of lung connective tissue and approaches to the therapy of lung disease.