Although the Andes are believed to have resulted mainly from crustal shortening, the shortening history remains debated and appears to require lateral (along-strike) crustal flow. Three-dimensional viscous flow modeling shows that, within geological uncertainties, the Andes may have been produced by either Neogene shortening alone or with significant pre-Neogene shortening. These scenarios require major along-strike crustal flow and predict significantly different histories of uplift and crustal motion.