Polymer or “plastics” solar cells have been an intensively studied area since the discovery of efficient electron transfer between polymers and fullerenes and the introduction of the bulk-heterojunction concept. The last few years have seen significant improvement in plastic solar cell performance through aggressive research on the regioregular poly(3-hexylthiophene) (RR-P3HT) : [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) system. The morphology of the system is controlled through two major strategies which have proven effective in improving the device efficiency—thermal annealing and solvent annealing (slow growth). In this Feature Article, we review the recent progress on this material system. A detailed discussion on thermal annealing and solvent annealing approaches to improve device performance is presented, including a comparison between the two strategies. The effects of these two approaches on improving polymer crystallinity, light absorption in the polymer, carrier transport, blend film nano-morphology, etc. are summarized. We also include a brief discussion on accurate measurement and characterization techniques for polymer solar cells to correctly determine the efficiency by applying spectral mismatch factors. Future directions and challenges on polymer solar cell development are also discussed.