This account focuses on the discovery and the rationalization of solvent effects, especially those of water, on synthetically useful radical reactions. Noteworthy advantages were observed during the course of our study on radical reactions in aqueous media. The established advantages of water as a solvent for radical reactions are (1) facile room-temperature atom transfer radical cyclization of allyl iodoacetate, (2) highly efficient construction of medium and large rings via radical cyclization of ω-alkenyl iodoacetate, (3) enhancement of intermolecular radical addition, and (4) the highest deuterium incorporation in the deuterative reduction of organic halides. The favorable solvent effects of water are mainly due to its extremely high polarity, strong hydrogen bonding ability, and lack of easily transferable hydrogen via a radical process.