Thy-1 (CD90) is a 25-37 kDa glycosylphosphatidylinositol (GPI) -anchored glycoprotein expressed on many cell types, including T cells, thymocytes, neurons, endothelial cells, and fibroblasts. Activation of Thy-1 can promote T cell activation, and this role of Thy-1 is reviewed elsewhere. Thy-1 also affects numerous nonimmunologic biological processes, including cellular adhesion, neurite outgrowth, tumor growth, migration, and cell death. In reviewing the nonimmunologic functions of Thy-1, we discuss the phenotype of the Thy-1 null mouse, signaling pathways modulated by Thy-1, the role of the GPI anchor in Thy-1 localization to lipid rafts and signaling, and regulation of Thy-1 expression. Thy-1 is an important regulator of cell-cell and cell-matrix interactions, with important roles in nerve regeneration, metastasis, inflammation, and fibrosis.