Formation of stable clusters in colloidal suspensions

The experimental evidences and available theoretical explanations on formation of stable clusters in colloidal suspensions are reviewed. The clusters form in the parameters range intermediate between that corresponds to a stable suspension built up by singlets and that causing the irreversible coagulation of the suspension. The stable clusters can appear as a result of a competition between a short range attraction and a long range repulsion between colloidal particles or due to reversible flocculation in the shallow secondary potential well.