Faithful transmission of chromosomes during mitosis is ensured by the spindle assembly checkpoint. This molecular safeguard examines whether prerequisites for chromosome segregation have been satisfied and thereby determines whether to execute or to delay chromosome segregation. Only when all the chromosomes are attached by kinetochore microtubules from two opposite spindle poles and proper tension is placed on the paired kinetochores does anaphase take place, allowing the physical splitting of sister chromatids. Recent studies have provided novel insights into the molecular mechanisms through which the spindle assembly checkpoint is regulated by both the attachment of chromosomes to kinetochore microtubules and the tension exerted on kinetochores.