The rate-limiting step of actin polymerization is the formation of an initial actin nucleus from which a filament can grow (see Figure 17-7a). In cells, this step is used as a control point to determine where actin filaments are assembled and what types of actin structures are generated (see Figures 17-1 and 17-4). Two major classes of actin-nucleating proteins, the formin protein family and the Arp2/3 complex, nucleate actin assembly under the control of signal transduction pathways. Moreover, they nucleate the assembly of different actin structures: formins lead to the assembly of long actin filaments, whereas the Arp2/3 complex leads to branched networks. Here we discuss each separately and see how the power of actin polymerization can drive motile processes in a cell. We then touch on the recent discoveries of new, specialized actin-nucleating factors.