This paper presents a literature review on the mechanisms and models of coalescence of fluid particles. For the mechanisms, five categories are summarized, namely, turbulence fluctuation, viscous shear stress, capture in turbulent eddies, buoyancy and wake interaction. The models for collision frequency and coalescence efficiency as well as contact and drainage times available in literature are reviewed thoroughly. The development and limitation of the existing models are studied and possible improvements are proposed. (C) 2010 Elsevier Ltd. All rights reserved.