An exquisite molecular machine, the mitotic spindle, organizes and separates chromosomes in dividing cells, thereby ensuring equal partitioning of the genetic material. Our lab is reconstituting spindle functions using pure components and applying new tools for manipulating individual molecules to uncover how this machine operates. By physically probing the interactions between microtubules, protein filaments that comprise the spindle, and kinetochores, the specialized structures on each chromosome to which microtubules attach, we are testing candidate mechanisms for the generation and control of chromosome movement. Having a better mechanistic understanding of the spindle promises to revolutionize the design of chemotherapeutics that target spindle components. Ultimately, it will also guide efforts to develop useful man-made nanomachines, which so far cannot match the remarkable abilities of naturally occurring protein machines.