In the preceding chapters the optical phenomena occurring in the photolithography process were discussed. Two other physical effects have to be accounted for, namely the various bake steps that determine the chemical properties of the resist and, most importantly, the development process of the resist since it converts the latent bulk image into the final resist profile. In the following the standard models for photoresist baking and development are presented. As mentioned before the chemical modeling of the resist lacks great uncertainty. Often, fitting is the only means to obtain some kind of model that is needed to close the gap in the overall simulation flow. For example, if physical models are available for all processes except for the post-exposure bake, it makes sense to use a fitting formula in the overall simulation. Although good agreement is usually obtained between measured and fitted data, this empirical approach cannot be used for predictive investigations due to the lack of any physical motivation. Both the bake steps and the development reaction are--from a physical point of view--the most complicated and least understood phenomena in the overall lithographic process.