A statistical theory of magnetization reversal in thin films for reversing fields less than the anisotropy field is presented in which it is assumed that reversal takes place by domain wall motion. The number of moving domain walls is determined by a simple statistical model which allows for the random nucleation of walls as the reversing field increases. Experimental data are presented in support of this theory.Two equations for the velocity of a domain wall in a thin film are presented; these arise since two alternative forms of the equation of motion of the magnetization vector are considered.