Publisher Summary The chapter discusses the protein folding and association: in vitro studies for self-organization and targeting in the cell. The chapter describes several techniques for the hierarchies of structure, stability, and folding. The chapter also discusses the mechanism of folding and association and the fundamentals of structure formation. It provides useful information for the study of folding of small single-domain proteins. In fact, after 20 years of in vitro protein folding, the practical application of denaturation/renaturation cycles in the downstream processing of recombinant proteins forced researchers in the field to ask questions that focused on the cellular aspects of folding. In case of the chaperones, the fundamental importance, beyond the stress response, has been clearly shown: (i) for the kinetic partioning among folding, association, and aggregation of oligomeric proteins; (ii) for protein targeting, e.g., translocation of mitochondrial proteins across membranes; and (iii) for processes involved in morphogenesis, such as growth and self-assembly of phages. The elucidation of the detailed folding mechanism of a given protein would require the complete description of the nascent (unfolded) and final native states, together with all the intermediates along the multistep folding path.