Actin-based motility processes are tightly linked to the rapid turnover of actin filaments. Factors that control the steady state of actin assembly, such as capping proteins and actin-depolymerizing factor/cofilin, directly affect motility. Actin-depolymerizing factor increases the treadmilling of actin filaments in vitro and in vivo. Cellular factors that are involved in linking initiation of barbed end assembly to cell signaling are being identified using Listeria monocytogenes and Saccharomyces cerevisiae as model systems.