The kinetics of order formation in symmetric diblock copolymers is investigated using a nonlinear dynamic equation. The equilibrium solution is the periodic pattern of the ordered state. The theory describes both the regime of weak and Strong segregation. In the latter case we find structure formation to proceed on two time scales : a rapid formation of kinks in the concentration profile is followed by a slow ordering of the kinks on to a periodic lattice. Explicit formulae for the temperature dependence of the relaxation times are derived. The results are found to agree qualitatively with experimental observations.