Oriented aggregation is a special case of aggregation that provides an important route by which nanocrystals grow, defects are formed, and unique, often symmetry-defying, crystal morphologies are produced. This growth mechanism involves the irreversible and crystallographically specific self-assembly of primary nanocrystals and results in the formation of new single crystals, twins, and intergrowths. This paper presents a molecular model for the mechanism of oriented aggregation. This model involves a rapid equilibrium for the association and dissociation of nanocrystals and an irreversible conversion of associated nanocrystals for producing oriented aggregates.