The degree of crystallinity, as well as the dimensions and orientation of crystalline domains, have important effects on the characteristics of organic electronic devices including photovoltaics, transistors, sensors, and light emitting diodes. While considerable effort has been devoted to controlled packing of conjugated organic materials on the molecular scale, and on deposition and annealing methods to tune crystalline structure, comparatively little work has focused on solution assembly of well-defined nanoscale structures that can subsequently be used as building blocks for device fabrication. Motivated by the desire to tailor structure and electronic properties of conjugated organic materials, our group has studied approaches to crystallization-driven assembly based on poly(3-hexyl thiophene) (P3HT). While P3HT is well known to form nanofibers upon crystallization from solution, we have focused on approaches to control the nucleation and growth of these nanostructures, as well as to assemble nanofibers bearing reactive groups that allow for crosslinking and chemical functionalization. In addition, we have discovered that P3HT serves as a crystal modifier during growth of small conjugated molecules, for example perylene diimides, thus providing a means to control crystal dimensions and to form hybrid crystal structures with anisotropic ambipolar charge transport characteristics.