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The first reports that nanostructured silicon emits radiation in the visible wavelength date back twenty years. This observation is interesting from a fundamental point of view and is potentially important for device engineering, as it points to the opportunity of combining microelectronics and optoelectronics on the same silicon chip. This combination is becoming increasingly attractive due to the intrinsic limitations that are being met by the microelectronics industry in its attempt to progressively reduce device features. In this review, we describe and compare the structure and properties of group IV nanocrystals (mostly silicon, with some references to germanium) grown by ion implantation and reactive pulsed laser deposition. Particular emphasis is placed on structural and spectroscopic characterization, as well as size-dependent optical properties. The origin of the luminescence signal and the role of defects and interfaces are discussed in detail.