Photonic crystals (PhCs) are periodically structured optical media offering the opportunity for spontaneous emission (SpE) to be strongly controlled in spatial terms (directions) or in absolute terms (rates). We discuss the application of this concept for practical light-emitting sources, summarizing the principles and actual merits of various approaches based on two- and three-dimensional PhCs. We take into consideration the numerous constraints on real-world light-emitting structures and materials. The various mechanisms through which modified photonic bands and band gaps can be used are first revisited in view of their use in light sources. We then present an in-depth discussion of planar emitters and enhanced extraction of light thanks to grating diffraction. Applications to conventional III-V semiconductors and to III-nitrides are reviewed. Comparison with random surface roughening reveals some common physical limitations. Some advanced approaches with complex structures or etched active structures are also discussed. Finally, the most promising mechanism to enhance the SpE rate, the Purcell effect, is considered. Its implementation, including through plasmonic effects, is shown to be effective only for very specific sources. We conclude by outlining the mix of physics and material parameters needed to grasp the relevant issues.