The cytoskeleton is built on a framework of three types of protein filaments: intermediate filaments, microtubules, and actin filaments. This chapter considers the structure and function of each of these protein filament networks. It begins with intermediate filaments, which provide cells with mechanical strength. The chapter explains how microtubules organize the cytoplasm of eukaryotic cells and form the hair like motile appendages that enable cells like protozoa and sperm to swim. It considers how the actin cytoskeleton supports the cell surface and allows fibroblasts and other cells to crawl. The chapter discusses how the actin cytoskeleton enables our muscles to contract. The contraction of muscle cells represents a highly specialized use of the basic components of the eukaryotic cytoskeleton. In skeletal muscle cells, repeating arrays of overlapping filaments of actin and myosin-II form highly ordered myofibrils, which contract as these filaments slide past each other.