Detailed structural information from hydrogen exchange and related experiments indicate the following three physical principles underlying protein folding pathways.(1) Proteins are made up of small cooperative unfolding/folding submolecular units known as foldons.(2) Proteins construct these foldon pieces to progressively build their final native states using the sequential stabilization principle where pre-formed foldons guide and template the subsequent foldons.(3) Ubiquitous optional misfolding errors can corrupt different naturally occurring on-pathway intermediates and cause intermediates to accumulate by inserting error-repair barriers at different points along the pathway.