Colloidal semiconductor nanocrystals (NCs) are metatstable objects prone to thermal and oxidative degradation driven by their large surface-to-volume ratios. The fabrication of practical electronic devices based on NC solids hinges on developing methods to prevent oxidation, diffusion, sintering and other undesirable physical and chemical changes to which these materials are susceptible. In this talk, I first describe systematic measurements of the room-temperature electron and hole field-effect mobilities of alkanedithiol-treated PbSe NC films as a function of NC size and the length of the alkane chain. These results establish a baseline for mobility trends in PbSe NC solids and have implications for fabricating high-mobility NC-based optoelectronic devices. Optical, electrical and photoelectron spectroscopy measurements are combined to monitor the room-temperature oxidation of films of PbSe NCs that are treated in solutions of short-chain thiols or carboxylic acids to produce electronically-coupled NC solids. I show that surface oxidation can be prevented by infilling NC films with thin (10-20 nm) Al2O3 layers deposited by low-temperature atomic layer deposition (ALD). ALD treatment of complete PbSe NC field-effect transistors yields high-performance devices that operate indefinitely in air. ALD infilling is a promising route to the preparation of stable, all-inorganic NC solids with tunable electrical properties, and may prove an important breakthrough in the fabrication of robust, high-efficiency quantum dot solar cells.