We summarize recent findings on the organization of the protein actin in eucaryotic cells. In particular we focus on how actin can be used to generate a vectorial force that is required for cell movement. These forces arise from protein molecules that recruit actin to the plasma membrane in such a manner that actin filaments extend outward from the cell body. This type of actin dependent force generation has been described in a nucleation-release model, which is one of several models currently being tested to explain actin dependent cell movement. Data in support of this model has arisen unexpectedly from studies of an intracellular bacteria, Listeria monocytogenes. This bacteria uses actin to propel itself during infection of eucaryotic cells. By studying Listeria movement, the roles of several eucaryotic actin interacting proteins have been identified. One of these is zyxin, a human protein that shares important structural and possibly functional properties with ActA, an actin dependent force generating protein of Listeria. We intend to test the function of these and other actin interacting proteins in a simplified system that should facilitate precise measurement of their properties of force generation in vitro.