The invention provides novel 8-azabicyclo[3.2.1]octane compounds of formula (I): wherein R1, R2, R3, and a are defined in the specification, or a pharmaceutically-acceptable salt thereof, that are antagonists at the mu opioid receptor. The invention also provides pharmaceutical compositions comprising such compounds, methods of using such compounds to treat conditions associated with mu opioid receptor activity, and processes and intermediates useful for preparing such compounds.