Silicon photonics is no longer an emerging field of research and technology but a present reality with commercial products available on the market, where low-dimensional silicon (nanosilicon or nano-Si) can play a fundamental role. After a brief history of the field, the optical properties of silicon reduced to nanometric dimensions are introduced. The use of nano-Si, in the form of Si nanocrystals, in the main building blocks of silicon photonics (waveguides, modulators, sources and detectors) is reviewed and discussed. Recent advances of nano-Si devices such as waveguides, optical resonators (linear, rings, and disks) are treated. Emphasis is placed on the visible optical gain properties of nano-Si and to the sensitization effect on Er ions to achieve infrared light amplification. The possibility of electrical injection in light-emitting diodes is presented as well as the recent attempts to exploit nano-Si for solar cells. In addition, nonlinear optical effects that will enable fast all-optical switches are described.