Oriented attachment (OA), or the non-random aggregation of crystallites to form single or twinned crystals, has been observed is a variety of different systems during crystal growth. OA is believed to underlie the growth of anisotropic and potentially useful nanostructures, such as wires and plates, as well as complex hierarchical nanostructures. However, its origins are poorly understood, l review insights into OR that we gained in two sets of molecular-dynamics simulation studies of titanium dioxide (anatase) nanocrystals. In the first set of studies, we focused on the role of intrinsic nanocrystal forces in facilitating nanocrystal alignment and aggregation in vacuum. These studies show that, although nanocrystal aggregation occurs in a predictable way, OA is not a common outcome. In a second set of studies, we used the ReaxFF reactive force field to study anatase nanocrystal aggregation in an aqueous (humid) environment. OA occurs in these studies and is mediated by adsorbed water and surface hydroxyls. The OA mechanisms that we find for anatase may be common to other aqueous metal-oxide systems. (C) 2014 Elsevier Ltd. All rights reserved.