Cytokinesis is essential for cell proliferation. Its failure leads to aneuploidy, which is often associated with cancer. In spite of its importance, the molecular mechanism of cytokinesis has not yet been fully clarified. We would like to understand cytokinesis more fully, in terms of how molecular machines assemble dynamically. The central spindle is a microtubule-based molecular assembly that forms between the segregating chromosomes during anaphase. During telophase, it associates with the ingressing cleavage furrow and matures into the midbody. These microtubule-based structures play crucial roles in all stages of cytokinesis from initiation to completion. We will address the following questions: 1) How is the central spindle/midbody assembled? 2) How does the central spindle/midbody contribute to the progression of cytokinesis? To address these questions, we have been combining various approaches including genetics in Caenorhabditis elegans , biochemistry and live observation in mammalian cultured cells and observation at the single molecule level by total internal reflection fluorescence microscopy (TIRF).