Renewable poly(lactic acid) (PLA) exhibits high strength and stiffness. PLA is fully biodegradable and has received great interest. However, the inherent brittleness of PLA largely impedes its wide applications. In this article, the recent progress in PLA toughening using various routes including plasticization, copolymerization, and melt blending with flexible polymers, was reviewed in detail. PLA toughening, particularly modification of impact toughness through melt blending, was emphasized in this review. Reactive blending was shown to be especially effective in achieving high impact strength. The relationship between composition, morphology, and mechanical properties were summarized. Toughening mechanisms were also discussed. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1051-1083, 2011