Silicon nanocrystals (SiNCs) synthesized by plasma-induced or high temperature processes (e.g., thermal disproportionation of hydrogen silsesquioxane at T > 1100 °C) display bright (photoluminescence quantum yield up to 70%) and long-lived luminescence (hundreds of μs), which can be tuned from green to red and near-infra-red spectral regions according to nanocrystal dimensions. The present review focuses on the parameters affecting the optical properties of these SiNCs, namely size, shape, surface, degree of crystallinity, and on a method to increase their brightness by functionalising SiNCs with dyes to build up a light-harvesting antenna. The final discussion presents some of the most recent examples of applications, which take advantage of the luminescence properties of SiNCs: energy conversion devices, sensors, and bioimaging probes.