The efficiency of protein folding suggests that this process proceeds through some intermediate states, raising the possibility of experimental observation of incompletely folded states of proteins. However, critical analysis of the observed partly folded stable states of proteins shows that they present either misfolded forms obtained under conditions inappropriate for folding, or partially unfolded states that retain folded the subpart of these molecules. This retained part, which unfolds last and folds first in a unfolding/refolding experiment, has a definite tertiary structure maintained by specific long-range interactions and can be isolated by fragmentation. Therefore, it can be regarded as a definite domain of the protein molecule. Provided the polypeptide chain of a small single domain protein does not become trapped in a misfolded form, its folding proceeds very rapidly, with all intermediates being transient and extremely unstable.