The spindle assembly checkpoint (SAC) is a critical quality control mechanism that ensures accurate chromosome segregation by delaying cell division until all chromosomes are stably attached to the spindle. This review focuses on three key questions: how unattached kinetochores generate the 'on' signal, how the SAC inhibits anaphase onset, and how kinetochore-microtubule attachment satisfies the SAC and extinguishes the on signal. Unattached kinetochores generate the SAC 'on' signal by recruiting proteins such as Bub1, Mad2, and Mad3 to form the mitotic checkpoint complex (MCC). The MCC then inhibits the anaphase-promoting complex (APC/C) by recruiting it to the MCC and preventing its activation. Once all kinetochores are stably attached to the spindle, the SAC is inactivated through processes like stripping, where the MCC is disassembled, and APC/C inhibitors are inactivated. The role of Mps1 kinase in promoting the template reaction for MCC assembly and maintaining SAC function is also discussed. The MCC is assembled from the Mad2-Cdc20 complex and the Mad3/BubR1-Bub3 complex, with Mad3 acting as a pseudosubstrate to inhibit substrate binding to the APC/C. The nature of the APC/C inhibitor is complex, with BubR1, not Mad2, being the key inhibitor. BubR1 inhibits the APC/C by blocking substrate binding, possibly through a conformational change or by occupying the substrate binding site. Finally, the review explores how the SAC 'on' signal is extinguished, including the role of stripping, phosphatases, and MCC disassembly. Stripping removes the SAC components from attached kinetochores, while MCC disassembly involves the action of p31comet and APC15, which disrupt the MCC and liberate Cdc20 to activate the APC/C.The spindle assembly checkpoint (SAC) is a critical quality control mechanism that ensures accurate chromosome segregation by delaying cell division until all chromosomes are stably attached to the spindle. This review focuses on three key questions: how unattached kinetochores generate the 'on' signal, how the SAC inhibits anaphase onset, and how kinetochore-microtubule attachment satisfies the SAC and extinguishes the on signal. Unattached kinetochores generate the SAC 'on' signal by recruiting proteins such as Bub1, Mad2, and Mad3 to form the mitotic checkpoint complex (MCC). The MCC then inhibits the anaphase-promoting complex (APC/C) by recruiting it to the MCC and preventing its activation. Once all kinetochores are stably attached to the spindle, the SAC is inactivated through processes like stripping, where the MCC is disassembled, and APC/C inhibitors are inactivated. The role of Mps1 kinase in promoting the template reaction for MCC assembly and maintaining SAC function is also discussed. The MCC is assembled from the Mad2-Cdc20 complex and the Mad3/BubR1-Bub3 complex, with Mad3 acting as a pseudosubstrate to inhibit substrate binding to the APC/C. The nature of the APC/C inhibitor is complex, with BubR1, not Mad2, being the key inhibitor. BubR1 inhibits the APC/C by blocking substrate binding, possibly through a conformational change or by occupying the substrate binding site. Finally, the review explores how the SAC 'on' signal is extinguished, including the role of stripping, phosphatases, and MCC disassembly. Stripping removes the SAC components from attached kinetochores, while MCC disassembly involves the action of p31comet and APC15, which disrupt the MCC and liberate Cdc20 to activate the APC/C.