The condensation fronts (snow lines) of water, carbon monoxide (CO)and other abundant volatiles in the midplane of protoplanetary disksaffect various aspects of planet formation and composition. CO, one ofthe major volatiles in disks, is believed to freeze out on dustgrains in the disk midplane when the temperature is below ~20K. Locating the CO snow line directly from millimeter CO data is achallenge since CO gas remains abundant in the warm atmosphere abovethe disk midplane, also exterior to the CO snow line. Using data fromthe Submillimeter Array (SMA) and Atacama LargeMillimeter/Submillimeter Array (ALMA), we have put strong constraintson the location of the CO snow line in the disks, using two methods:(1) multi-transition observations of CO and its isotopologues, and(2) observations of the trace species like N2H+ that are abundant only whereCO freezes out. Finally I will discuss how we can use chemical imagingto locate other major snow lines in disks.