Efficient development of stable formulations of protein pharmaceuticals requires an intimate knowledge of the protein and its chemical and physical properties. In particular, understanding the mechanisms by which a protein could degrade is critical for designing and testing formulations. This review describes the major pathways by which proteins can degrade, including denaturation, aggregation, oxidation, and interfacial damage. The methods to detect the degradation are covered, along with generalized strategies to retard or prevent each type of decomposition. Without an appreciation of the current best practices for devising stable formulations, the formulation process will be neither efficient nor optimal.