Publisher Summary This chapter focuses on simple activity coefficient models for polymers that can be applied to a wide range of applications. These are group-contribution based (UNIFAC) models that account for some special effects in polymer systems such as free-volume differences. A summary of some rules of thumb for predicting polymer-solvent miscibility, with focus on the screening of solvents for polymers is presented. These rules are based on well-known concepts of thermodynamics (activity coefficients, solubility parameters) and some specific ones to polymers. The chapter discusses some of the concepts involved. It can be roughly said that a chemical (1) is a good solvent for a specific polymer (2), or in other words the two compounds are miscible if one (or more) of the following “rules of thumb” are valid. The chapter also explains the free-volume activity coefficient models. The rules of thumb based on solubility parameters are widely used. The starting point (in their derivation / understanding) is the equation for the Gibbs Free-Energy of mixing. The chapter also discusses (1) the rule of thumb based on the infinite dilution activity coefficient, (2) the rule of thumb based on the Flory–Huggins model, (3) the free-volume activity coefficient models, (4) extension of free-volume models to gas solubilities in elastomers, and others.