Polymer crystallization leads to spontaneous formation of a nanocomposite structure in which plate-like crystals that give strength are separated by noncrystalline material that confers toughness.  This highly desirable combination of qualities explains why semicrystalline polymers account for over two thirds of the 170 million metric tons of polymers produced annually.  Their material properties can be tuned over a wide range by processing because their nano- and  microstructure formation is kinetically controlled.  Designing processing flows has historically depended on trial and error.  Molecular understanding has been elusive, but we are now making significant progress through recent advances in both instrumentation and materials.  During flow, we observe the creation of the oriented precursors that accelerate solidification and template morphology.  Model materials allow us to pinpoint the macromolecular dynamics that control precursor formation.  This lecture will present our latest insights into polymer crystallization and their implications for materials design.