Checkpoints reduce the frequency of errors in cell division by delaying the progress of the cell cycle until certain processes are complete. The spindle-assembly checkpoint prevents the onset of anaphase until a bipolar spindle is present and all chromosomes are attached to the spindle. Evidence from yeast and mammalian cells suggests that kinetochores are at least one source of the signal that stops the cell cycle. Recent studies in budding yeast have begun to define the signal-transduction pathway involved in the spindle-assembly checkpoint, but details of the endpoint of the pathway, where these signals interact with the cell-cycle machinery, remain to be characterized.