Nanocrystals exhibit a wide range of unique properties (e.g., electrical, optical, optoelectronic, catalytic, etc.) that depend sensitively on their size and shape, and are of both fundamental and practical interest. Breakthrough strategies that will facilitate the design and synthesis of a large diversity of nanocrystals with different properties and controllable size and shape in a simple and convenient manner are of key importance in revolutionarily advancing the use of nanocrystals for a myriad of applications in lightweight structural materials, optics, electronics, photonics, optoelctronics, magnetic technologies, sensory materials and devices, catalysis, drug delivery, and biotechnology among other emerging fields. In this talk, I will elaborate four general and robust strategies for crafting a rich variety of functional 0D 1D, shish-kebab, and Janus nanocrystals with precisely controlled dimensions (e.g., plain, core/shell and hollow nanoparticles; plain and core/shell nanorods; nanotubes; Janus nanoparticle, etc.) by capitalizing on a set of rationally designed unimolecular star-like, bottlebrush-like, worm-like, and Janus block copolymers, respectively, as nanoreactors. These four strategies are effective and able to produce oil-soluble and water-soluble monodisperse nanocrystals, including metallic, ferroelectric, magnetic, luminescent, semiconductor, perovskite, and their core/shell structures, which represent a few examples of the kind of nanocrystals that can be produced using these techniques. The applications of these functional nanocrystals in energy conversion and storage (e.g., dye-sensitized solar cells, perovskite solar cells, photocatalysis, LEDS, thermoelectrics, ferroelectrics, and batteries) will also be discussed.