This appendix recalls basic notions on modules, abelian resolutions, and homology.

This section gives a general overview of abelian model structures and their homotopy categories. It is also meant to be a survey of the most fundamental examples of such homotopy categories. These include the chain homotopy category of a ring, the derived category of a ring, and the stable module category of a quasi-Frobenius (or Iwanaga–Gorenstein) ring.

This chapter studies cotorsion pairs and abelian model structures on chain complexes over additive and exact categories. Fundamental properties of the chain homotopy relation and contractible chain complexes are developed before it is shown that the category of chain complexes over any additive category is a Frobenius category. This determines an abelian model structure whose homotopy category is the classical chain homotopy category of complexes. We then examine general properties of abelian model structures on chain complexes. Formal (Quillen) homotopy categories of complexes are identified with triangulated subcategories of the classical chain homotopy category. The homotopy categories are also identified with Verdier quotients of the classical chain homotopy category. The end of the chapter constructs abelian models for derived categories. The first construction is an abelian model structure for the derived category of any exact category with coproducts, kernels, and a set of (small) projective generators. Second, it is shown how any complete hereditary cotorsion pair on a Grothendieck category lifts to a hereditary abelian model structure on the associated category of chain complexes.