Fluorescent nanodiamonds are a promising and attractive platform for quantum technology due to the combination of outstanding properties of diamond color centers and the nanoscale size of crystals. It allows their integration with different types of photonic structures for quantum information and communication purposes as well as usage for sensing in various environments, including harsh conditions and biological systems. This chapter will discuss the fabrication methods of fluorescent nanodiamonds, the optical properties and specific features related to their nanosize and surface, and some key applications.