Design criteria for ideal foam nucleating agents, also called nucleants, are identified. Nucleants conforming to the criteria are expected to produce desirable foams with a large number density and narrow size distribution of bubbles. The criteria are based on an analysis of the thermodynamics and kinetics of coupled nucleation and growth. Based on this work, ideal nucleants have four characteristics. Nucleation on ideal nucleants is energetically and kinetically favorable relative to homogeneous nucleation. Ideal nucleants also have uniform geometry and surface properties, they are easily dispersed, and they are plentiful enough to overwhelm the contributions of homogeneous and unintentional heterogeneous nucleation. (C) 2004 Elsevier Ltd. All rights reserved.