Cell division is essential to the propagation of life. During cell division, mitotic spindles segregate duplicated chromosomes with high fidelity. Spindle structure depends on the proper polar organization of microtubules, which have biochemically distinct plus and minus ends. Interactions between microtubules, motor proteins, and crosslinkers organize the spindle into a bipolar array in which sister chromatids attach via their kinetochores to a subset microtubules within the spindle structure. We are working to understand how the mitotic spindle assembles into the correct structure. We combine biophysical modeling with quantitative characterization of mitosis in fission yeast, a cell type that is simple enough to be understandable and amenable to experimental modification. Recent projects have focused on spindle length instabiliity and self assembly of a bipolar spindle.