Process windows are frequently generated from simulated aerial image profiles by use of a threshold model for the resist process, an assumption which is not accurate for many processes. In this paper, we present new computationally efficient methods for incorporating the effects of resist processing into simulated images. The First Order Model of development leads to the simple result that the resist linewidth W is smaller than the threshold linewidth Wthresh by an amount (Delta) W approximately equals 2 [ln(D(gamma) s)-1]/((gamma) s), where D is the resist thickness, (gamma) is the resist process non-linearity and s is the log-slope of the image. A Second Order Model based on a segmented development path is also presented. These models allow the prediction of resist linewidths based on calculated image profiles for any wet developed process: optical, X-ray or e-beam lithography, both positive and negative resists. The predictions of these models show good agreement with full PROLITH/2 resist profile simulations. We have also incorporated a Fickian diffusion of the intensity profile into our model, to account for acid diffusion, stepper vibration, lens aberrations, and other effects which reduce process resolution. Experimental process windows are well matched by such models, and are significantly different than threshold model predictions.