Here we review major breakthroughs in the rapidly developing research area of nanomaterial synthesis in supercritical fluids focusing on both the fundamental science and processing advantages in the application areas of separations, assembly and scale-up of synthesis. Density-tunable control of the solvation strength, along with favorable interfacial tensions, wetting, and transport properties make supercritical fluids (SCFs) unique and potentially superior solvent media for the synthesis and processing of nanoscale metals and semiconductors. The steric repulsion between particle cores may be tuned by adjusting the solvent density to control the nanocrystal size and polydispersity during single-phase synthesis, and to achieve size-selective nanocrystal precipitation. Compressed solvents at the vapor pressure can also be used to deposit nanocrystal monolayers at controlled evaporation rates without dewetting at higher rates than conventional solvents. The high synthetic temperatures accessible in SCFs can be used to promote the crystallization of covalent nanomaterials, while maintaining a reaction environment capable of solvating reactants and capping ligands. This allows semiconductor nanocrystals exhibiting size-dependent photoluminescence to be synthesized at temperatures well above the boiling point of conventional solvents. Finally, gold nanocrystal seed particles can be used to synthesize bulk quantities of defect-free silicon and germanium nanowires, several micrometers in length.