The phase behavior of organically passivated 20-75 Angstrom diameter Ag and Au nanocrystals is investigated by examining surface-area isotherms of Langmuir monolayers and transmission electron micrographs of Langmuir-Blodgett (LB) films. The effects of temperature, organic passivant chain length, and nanocrystal size and composition are studied. Three distinct types of phase behavior are observed and may be classified in terms of the extra (conical) volume (V-e) available to the alkyl capping group as it extends from a nearly spherical metal core. For V-e > 350 Angstrom(3), the phase diagram is dominated by extended, low-dimensional structures that, at high pressures, compress into a two-dimensional foamlike phase. This behavior is rationalized as originating from the interpenetration of the ligand shells of adjacent particles. For V-e < 350 Angstrom(3), dispersion attractions between the metal cores dominate particle condensation. For 350 Angstrom(3) > V-e > 150 Angstrom(3), the particles condense to form closest packed structures, which, for sufficiently narrow particle size distributions, are characterized by crystalline phases. For V-e approximate to 30 Angstrom(3), the particles irreversibly aggregate into structures similar to those expected from a diffusion-limited-aggregation (DLA) model. Optical properties of certain LB films of the closest packed phases are reported.