We study well-posedness for the initial value problem associated to the Benjamin equation and the periodic Benjamin equation. Global results are established for data in L-R(R) and L-2(T), respectively. We apply the recent theory, developed by Kenig, Pence, and Vega and Bourgain, to deal with low-regularity data for the initial value problem associated to the Korteweg-de Vries equation. (C) 1999 Academic Press.