Tricyclic compounds containing a cyclopropyl carboxylic acid or carboxylic acid derivative (e.g. amide) fused to a bicyclic ring, including pharmaceutically acceptable salts and prodrugs thereof, are agonists of G-protein coupled receptor 40 (GPR40) and are useful as therapeutic compounds, particularly in the treatment of Type 2 diabetes mellitus, and of conditions that are often associated with this disease, including obesity and lipid disorders, such as mixed or diabetic dyslipidemia, hyperlipidemia, hypercholesterolemia, and hypertriglyceridemia.