Kinetochores are multiprotein complexes that assemble on centromeric DNA and mediate the attachment and movement of chromosomes along the microtubules (MTs) of the mitotic spindle. This review focuses on the simplest eukaryotic centromeres and kinetochores, those found in the budding yeast Saccharomyces cerevisiae. Research on kinetochore function and chromosome segregation is focused on four questions of general significance: what specifies the location of centromeres? What are the protein components of kinetochores, and how do they assemble a MT attachment site? How do MT attachments generate force? How do cells sense the state of attachment via the spindle assembly checkpoint?