The two latest volcanic pulses in western Panama occurred in the middle Miocene and Quaternary. At least two volcanoes have been active in historic times. A period of relative quiescence (almost 10 m.y) separates these pulses. The most recent volcanism has a calc-alkaline character. Geochemical trends in individual centers are consistent, but the scatter in trace elements suggests a complex magma evolution. K2O content, LIL concentrations, and volcanic edifice height increase westward along the western Panamanian arc, providing a pattern which is bilaterally symmetric to that in northern Costa Rica. This pattern may result from processes related to the subduction of the Cocos Ridge (a thick crustal segment of the Cocos plate). Previous workers believed that subduction below Panama ceased some 9 Ma. Calc-alkaline volcanism, subdued but active seismicity, well developed Bouguer gravity anomalies, and folded-thrusted young sediments along the Panama Trench, however, suggest active subduction. Reactivation of northeast-directed subduction probably occurred in the Zanclean (3.4–5.3 Ma) and may be due to “unplugging” of the trench by westward migration of the Cocos Ridge choke zone, increased slabpull, and/or renewed (Quaternary) spreading along the Sandra rift system. The “slabpull” hypothesis is supported by significant thinning of the oceanic crust and high crustal heat flow near the trenches. Seismic evidence (although scarce) suggests northeast, low-angle, subduction below Panama — a subduction mode comparable to that of the Rivera platelet on the other extreme of the Central American orogen.