After a rapid increase in temperature across the deconfinement temperature T-d, pure gauge theories exhibit unstable long wavelength fluctuations in the approach to equilibrium. This phenomenon is analogous to spinodal decomposition observed in condensed matter physics, and also seen in models of disordered chiral condensate formation. At high temperature, the unstable modes occur only in the range 0 less than or equal to k less than or equal to k(c), where k(c) is on the order of the Debye screening mass m(D). Equilibration always occurs via spinodal decomposition for SU(2) at temperatures T > T-d and for SU(3) for T much greater than T-d. For SU(3) at temperatures T greater than or similar to T-d, nucleation may replace spinodal decomposition as the dominant equilibration mechanism. Monte Carlo simulations of SU(2) lattice gauge theory exhibit the predicted phenomena. The observed value of k(c) is in reasonable agreement with a value predicted from previous lattice measurements of m(D). (C) 2000 Elsevier Science B.V. All rights reserved.