A fundamental challenge in the field of catalysis is the development of sustainable and efficient methods for the activation of molecules. One approach for the activation of organic molecules that has recently received much attention is photoredox catalysis with visible light. On the other hand, the application of aryl radicals in organic synthesis is challenging, but very useful. This thesis describes the generation of aryl radicals and their application in organic synthesis, such as the direct arylation of heteroarenes, the synthesis of benzothiophenes, phenanthrenes and the amino arylation of alkenes using visible light photoredox catalysis. In addition, the synthesis of substituted tetrahydroisoquinolines is described.