The experimental evidence and theoretical explanations of stable cluster formation in colloidal suspensions are reviewed. The clusters form in the intermediate range between a stable suspension built up by singlets and the irreversible coagulation or gelation of the suspension. The stable clusters develop as a result of a balance between competing short range attraction and long range repulsion between colloidal particles or due to reversible flocculation in the shallow secondary potential well. Heteroaggregation in binary colloids can also result in formation of stable clusters.