The development of quantum technologies from the bottom-up requires engineering and control at the level of single quanta. In this thesis. we explore the engineering of a particular quantum system: optically active defects in diamond. known as color centers. Color centers have a variety of advantageous characteristics, including long-lived quantum phase coherence in their electron spin degrees of freedom, available high-fidelity control and readout at the single-spin level, and strong interactions with other quantum systems such as photons. In addition, color centers can be controllably created and their local environment shaped through semiconductor fabrication processes that form a toolkit for engineering quantum devices on the nanoscale. We apply this toolkit to several challenges: the development of sensitive spin-based magnetometers with high spatial resolution, the production of tightly-confined spin clusters for use as small quantum computers, quantum memories or magnetic resonance sensors, and the integration of spin quoits with optical nanostructures as spin-photon interfaces. In parallel. we develop metrology techniques to characterize our quantum devices, and apply these techniques to the problem of wide-field strain imaging in polycrystalline diamond. The devices shown here are building blocks for the development of future scaled quantum technologies, while the engineering and metrology toolkit we developed has direct application to the quickly growing field of semiconductor color center science. (Copies available exclusively from MIT Libraries, libraries.mit.edu/docs - docs mit.edu).