Solution phase syntheses and size-selective separation methods to preparesemiconductor and metal nanocrystals, tunable in size from ∼1 to 20 nm andmonodisperse to ≤5%, are presented. Preparation of monodispersesamples enables systematic characterization of the structural, electronic, andoptical properties of materials as they evolve from molecular to bulk in thenanometer size range. Sample uniformity makes it possible to manipulatenanocrystals into close-packed, glassy, and ordered nanocrystal assemblies(superlattices, colloidal crystals, supercrystals). Rigorous structuralcharacterization is critical to understanding the electronic and opticalproperties of both nanocrystals and their assemblies. At inter-particleseparations 5–100 Å, dipole-dipole interactions lead to energytransfer between neighboring nanocrystals, and electronic tunneling betweenproximal nanocrystals gives rise to dark and photoconductivity. At separations<5 Å, exchange interactions cause otherwise insulating assemblies tobecome semiconducting, metallic, or superconducting depending on nanocrystalcomposition. Tailoring the size and composition of the nanocrystals and thelength and electronic structure of the matrix may tune the properties ofnanocrystal solid-state materials.