Some of the most useful reactions in synthetic organic chemistry involve the addition of reagents across the double bonds of olefins. These reactions, which can occur by a variety of mechanisms involving electrophilic, nucleophilic or radical intermediates, have been the subject of numerous mechanistic studies. Electrophilic additions involve polar reagents, and in general the direction of addition follows Markownikoff’s rule. Nucleophilic additions require rather special and drastic conditions unless the olefin contains strong electron-withdrawing substituents. Radical reactions are very general in scope, but are markedly influenced by light, oxygen, peroxides, and various inhibitors. The direction of addition in free-radical reaction is commonly the opposite of that encountered in the electrophilic ionic (Markownikoff) additions. Hence these free-radical additions have been referred to as Anti-Markownikoff, abnormal or Karasch addition reactions. This chapter is concerned with the synthetic aspects of Kharasch addition reactions that result in the formation of a new carbon-carbon bond.