The formation of higher-molecular-mass species due to non-covalent adherence of smaller species. Especially for proteins, aggregation is a form of denaturation in which non-polar surfaces of secondary structures, e.g. those of α-helices and β-sheets that normally form intramolecular interactions and are buried within the interior of the protein, are allowed to interact intermolecularly and to form multimolecular forms that are sometimes insoluble. Aggregation is contrasted with oligomerization, the normal interaction of native, correctly folded, proteins into higher-order multimers.