Biogranulation that includes anaerobic and aerobic granular sludge processes is a promising biotechnology for wastewater treatment. The formation and structure of biogranules are associated very closely with cell hydrophobicity. This paper therefore attempts to review the essential role of cell hydrophobicity in the formation of granular sludge. Cell hydophobicity could be induced by culture conditions, and in turn initiates cell-to-cell aggregation that is a crucial step towards biogranulation. In this paper, the factors that may influence cell hydrophobicity were discussed, while a thermodynamic interpretation of cell hydrophobicity was also presented. Meanwhile, a selection pressure-based inducing strategy for cell hydrophobicity was further proposed. It appears that cell hydrophobicity is a triggering force of biogranulation, and high cell hydrophobicity seems to be a prerequisite of biogranulation. However, the knowledge regarding the role of cell hydrophobicity in biogranulation process is far from complete. Some future research niches are then outlined. (C) 2004 Elsevier Inc. All rights reserved.