Lower extremity peripheral artery disease (PAD) affects 8.5 million men and women in the United States and more than 200 million people worldwide. People with PAD have a two- to threefold increased rate of cardiovascular events and all-cause mortality compared to people without PAD. People with PAD also have greater functional impairment and faster functional decline than people without PAD. The ankle brachial index (ABI) is a reliable, sensitive, and highly specific noninvasive test for PAD and can be used to diagnose and assess the severity of PAD. This chapter provides an overview of the role of the ABI in diagnosing PAD and in assessing risk of cardiovascular outcomes, lower extremity outcomes, and functional decline.