The term “nanodiamond” is broadly used for a variety of diamond-based materials at the nanoscale (lengths in the scale of ~1 to 100 nm), including pure-phase diamond films, diamond particles, and their structural assemblies, such as loosely bound particle agglomerates or particles incorporated into other material matrices. The recently fabricated one-dimensional (1-D) diamond nanorods (DNR) and 2-D diamond nanoplates complete the set of possible dimensionalities of these structures. There is a special class of nanodiamond material called “ultrananocrystalline” diamonds (UNCD), the basic diamond constituents whose length is of the order of a few nanometers. They are called “ultra-nanocrystalline,” to distinguish them from other diamond-based nanostructures with characteristic sizes above ~10 nm. Among UNCD materials, the most promising for nanotechnological applications in the immediate future are UNCD particles produced by detonation of carbon explosive materials (characteristic size of primary particles ~5 nm) developed in the former USSR in the 1960s, and pure-phase UNCD films produced by chemical vapor deposition (CVD) (characteristic size of grains 2 to 5 nm) recently developed in the Argon National Laboratory [1]. UNCD particles and films possess unique properties, and have different niche applications. These two technologically important materials have been the focus of several recent reviews [1-4], monographs [5-7], as well as recent nanodiamond conferences [8-10], and will be major topics of this chapter.