The properties of many technological important materials are controlled by the presence of high number densities of microstructural features that have characteristic dimensions below 5 nm, such as ultrafine precipitates, nanoclusters, Guinier-Preston zones, and nanovoids. The various standard methods of atom probe tomography (APT) analyses of these features often result in different estimates of their compositions due primarily to the difficulty of defining the position of the interface, i.e., which atoms belong to the feature, which to the surrounding matrix phase, and the error associated with the small numbers of atoms. Some recent advances in APT characterization of nanoclusters and nanovoids will be presented. Research sponsored by the Materials Sciences and Engineering Division, Office of Basic Energy Sciences, US Department of Energy. Atom probe tomography (MKM) was supported by ORNL’s Shared Research Equipment (SHaRE) User Facility, which is sponsored by the Office of Basic Energy Sciences, US Department of Energy.