We discuss the validity of Langer's picture of homogeneous nucleation~1. Our approach is based on a coarse-grained free energy that incorporates the effect of fluctuations with momenta above a scale k. The nucleation rate I=A_kexp(-S_k) i sexponentially suppressed by the action S_k of the saddle-point configuration that dominates tunnelling. The factor A_k includes a fluctuation determinant around this saddle point. Both S_k and A_k depend on the choice of k, but, for 1/k close to the characteristic length scale of the saddle point, this dependence cancels in the expression for the nucleation rate. For weakly first-order phase transitions or in the vicinity of the spinodal-decomposition line, a large pre-exponential factor A_k compensates the exponential suppression exp(-S_k) and the standard nucleation picture breaks down. We also discuss radiatively induced first-order phase transitions. The saddle-point expansion fails again, as in this case A_k leads to an additional suppression of the nucleation rate comparable to the exponential one.
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