Control of molecular size and molecular weight distribution is necessary so that the resulting polymers have good processability and have specific properties required for their intended applications. Chain transfer is the process of regulating and controlling molecular weight. Some experts consider this process of chain transfer as the fourth process in polymerizations, along with initiation, propagation and termination. Chain transfer was first observed in radical polymerizations but occurs in ionic polymerizations also. Alkyl thiols (Mercaptans) have been recognized to be one of the most efficient chain transfer agents in polymerizations that produce polystyrene, styrene-butadiene rubber, ABS terpolymers, polymethacrylates (e.g. PMMA), polyacrylates and other vinyl-type of polymers. Chain transfer can also be used to make low molecular weight polymers (telomers), introduce branching and cross-linking. t-Dodecyl mercaptan (TDM) and n-dodecyl mercaptan (NDM) are examples of the two most common chain transfer agents. In many polymerizations, TDM is the choice due to the fact that its Chain Transfer Constant (CT) is an aggregate of different numbers since it is a mixture of several tertiary mercaptans of carbon number C10 to C13. NDM, on the other hand, is a pure C12 mercaptan with a higher CT, thus giving a narrower molecular weight distribution. Mechanism of the various reactions involved in chain transfer will be discussed. Some strategies involving the use of chain transfer agents-mercaptans in particular, for improving thermal stability of the polymers, increasing branching, providing enhanced cross-linking, will also be discussed.