Publisher Summary This chapter discusses the spindle assembly checkpoint in budding yeast. As cells proceed through cycles of growth and division they are often challenged by environmental insult and injury as well as spontaneous errors in cellular processes. The response to cell damage requires that cells sense the damage, arrest the cell cycle, and attempt to repair the lesion. This general process is known as checkpoint control and is a key mechanism cells employ to ensure accuracy and fidelity during cell division. Work demonstrates that a checkpoint regulates mitosis to ensure the faithful construction of the mitotic spindle and the proper segregation of the chromosomes. . In response to spindle depolymerization or incorrect attachment of chromosomes to microtubules, the spindle assembly checkpoint arrests cells at metaphasee. By preventing the onset of anaphase until spindle assembly and chromosome alignment are complete, the checkpoint helps to ensure that each daughter cell will receive an equal chromosomal complement.