Cover Photograph: Thermal noise is routine in the molecular world. Unsurprisingly, nature has not only adapted to it but found ways for its utilization. Actin polymerization, as well as many other cellular processes, is not a smooth process of lengthening or shortening of actin filaments but a random process vulnerable to thermal fluctuations. Fluctuation-driven subunit exchange between monomeric and polymeric actin pools, also called exchange diffusion, indirectly couples energy of ATP hydrolysis by polymeric actin to the energy of actin polymerization. This provides a basis for regulation of actin filament growth by an actin-binding protein profilin. See the article by Elena Yarmola and Michael Bubb in this issue. The authors wish to thank Reuben Judd from their laboratory, who provided significant help with preparation of the figure.