This chapter reviews the theoretical aspects of nanoparticles (NP) - Stimulated Emission Depletion (STED)—the use of hybrid fluorescent labels consisting of a fluorescent emitter and a metal NP in STED nanoscopy. It shows that this technique has great promise in allowing for a significant improvement in all aspects of the imaging performance of STED nanoscopy, namely allowing low STED power, high resolution, or improved photostability. This variety allows one to design an NP to suit the imaging requirements and object sizes. The proposed technique relies on challenging yet demonstrated NP fabrication technology for fluorescence microscopy. In addition, it is complementary to the improvements made to STED nanoscopes; thus, it is compatible with any STED mode of operation such as continuous-wave and time-gated STED modes. A proof-of-principle experiment has confirmed the NP-STED concepts using core–shell nanostructures. The chapter examines the dependence of the improved performance on nanoshell size. Larger nanoshells provide better improvement in imaging performance.