We have seen that actin filaments are assembled into a wide variety of different arrangements and that many associated proteins nucleate actin assembly and regulate filament turnover. Dozens of proteins in a vertebrate cell organize these filaments into diverse functional structures. Here we discuss just a few of these proteins, giving examples of typical types of actin cross-linking proteins found in cells as well as proteins involved in making functional links between actin and membrane proteins. One fascinating problem, about which very little is known, is how cells assemble different actin-based structures within the cytoplasm of the same cell. Some of this organization must be due to local regulation, a topic we come to at the end of the chapter.