This section, at first glance, may appear unrelated to our exposition of single chain statistical thermodynamics. This is because the focus is no longer on conformations of single chains or quantities like the end-to-end distance or the radius of gyration. Instead we focus on the combinatorial or packing entropyPacking entropy of many chains in a certain volume as well as on their free enthalpy. In the following we develop a simple mean field theory on the lattice, which nevertheless can be applied to blends, solutions, and networks. There are cases when our results will not be correct and will have to be revised. However, the lattice approach is certainly worthwhile, since it provides an overview without immediately blinding us with complexity.