Silicon is the most important semiconducting material of the microelectronic industry. Bulk silicon does not exhibit good optical properties, however in the late 1980s good emission was observed and studied in a silicon-based material, porous silicon. Since then, a variety of luminescent silicon nanostructures have been investigated, but different results and interpretations have been reported in the literature regarding the origin of the luminescence of these structures. This Faraday Discussion explores new methodologies to synthesize and characterise luminescent silicon nanostructures, from porous silicon to nanocrystals and nanorods. Attention is devoted to the most promising applications of these systems in the fields of bioimaging, sensing and energy conversion (e.g., OLED and luminescent solar concentrators). Wet, dry, chemical, physical, thermal, out-of-equilibrium formation paths are related to the physics of the produced nanostructures and to the role of the matrix (interface) in which they are embedded. In this volume the topics covered include:Synthesis and functionalisation of silicon nanostructuresOptical and electronic properties: from theory to experimentsSilicon nanostructures for sensing and bioimagingSilicon nanostructures for energy conversion devices