An overview of methodsf or the initiation of radical chain reactions by specific initiator compounds, which generate radicals, is given. These can be utilized to initiate any kind of radical chain reactionb yt ransforming substrates into the desired radical intermediates. Azo initiators,p eroxides, nitroxides, trialkylboranes, dialkyl zinc compounds, and type Ip hotoinitiators are discussed, as well as methods of redox-and sonochemical initiation. Methods of direct radical formation from the substrates, such as photoredoxc atalysis or high-energyi rradiation, are not included. The focus of this review lies on rather "low" temperatures in the range of 50 8Cd own to À78 8C, which can be useful to achievem ore selectiver eactions. Illustrative applications of such radical chain initiatorsi navariety of reactions are discussed, including stereoselective ones and polymerizations.Scheme1.Generalmechanism for radical chain reactions transforming substrates (S) into product (P) via intermediate radicals (iR), being initiatedw ith radicals generated from molecularinitiatorsX -Y.Scheme2.Classification of radicalgeneration from initiator molecules X-Y (D:thermal energy;h n:photochemically;)))): sonochemically).Scheme19. Applications of type-I photoinitiators in organics ynthesis.Scheme20. Initiation of athiol-enereaction with N-phenyltriazolinedione 40.Scheme21. Sonochemical initiation of radical reactions.