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Photoredox catalysis is a tool enabling a wide variety of chemical reactions with high selectivity under mild conditions of visible light. In this Review, we summarize recent experiments which use heterogeneous, covalent, metal-free semiconductors with adjustable reactivity to drive such reactions. This class started with mesoporous graphitic carbon nitride, then continued with poly(heptazine imides), but is meanwhile extended to other polymers and solid-state organics with conjugated electron system. Because of the high thermal and chemical stability, as well as adjustable conduction and valence band positions, the reaction space could be expanded to many organic reactions, such as photocatalytic synthesis of organosulfur compounds, or unconventional halogenation and cyanation reactions. Performance of carbon nitrides and homogeneous systems in certain reactions was compared in the present Review.