Gremlin is an extracellular glycoprotein that was first identified over a decade ago through its important role in embryonic development, in which it acts as an antagonist of bone morphogenetic protein actions. It plays a critical role in the development of normal airways and the pulmonary circulation in the embryo. More recently, considerable evidence has been presented for a role for gremlin in the pathogenesis of lung diseases, particularly pulmonary hypertension and idiopathic pulmonary fibrosis. The purpose of this article is to review this evidence, consider the potential mechanisms and multicellular actions by which gremlin contributes to disease pathogenesis, and suggest future avenues of research.