The spindle assembly checkpoint (SAC) is a critical quality control mechanism that ensures accurate chromosome segregation during mitosis and meiosis. It delays cell division until all chromosomes are correctly attached to the spindle microtubules via their kinetochores. When kinetochores are not properly attached, the SAC is activated, blocking cell cycle progression. Once all kinetochores are stably attached, the SAC is inactivated, allowing cell division to proceed. This review summarizes recent advances in understanding how the SAC signal is generated, how it blocks cell cycle progression, and how it is extinguished. Eukaryotic cells face unique challenges in ensuring accurate genome transmission due to the separation of genome replication and segregation. Sister chromatid cohesion, mediated by cohesin, ensures that replicated sister chromatids remain attached until all chromosomes are correctly aligned on the spindle. The SAC ensures that this cohesion is maintained until all chromosomes are properly attached, preventing premature separation. The SAC is a complex network involving proteins such as Mad1, Mad2, Mad3, Bub1, Bub3, and BubR1. These proteins are recruited to kinetochores in a stepwise manner, with Bub1 playing a key role in recruiting downstream SAC components. The Mad2 template model explains how unattached kinetochores generate the SAC signal, with Mad2 adopting two conformations (open and closed) to facilitate the formation of the mitotic checkpoint complex (MCC). The MCC then inhibits the anaphase promoting complex (APC/C), preventing premature anaphase onset. Mps1, a protein kinase, is essential for recruiting the Mad1–C-Mad2 core complex to kinetochores and for promoting the template reaction. The role of Mps1 is further supported by its interaction with the spindle matrix, which facilitates efficient kinetochore loading. The SAC signal is extinguished when microtubules attach to kinetochores, leading to the removal of the MCC and the release of Cdc20, allowing the APC/C to activate. The SAC is also regulated by the interaction of BubR1 with Cdc20, with BubR1 containing two conserved KEN boxes that are essential for SAC function. The APC/C inhibitor is primarily the MCC, which is assembled from the Mad2–Cdc20 complex and the Mad3/BubR1–Bub3 complex. The SAC signal is extinguished through mechanisms such as stripping, which removes the MCC from kinetochores, and through the inactivation of existing inhibitory complexes. In summary, the SAC ensures accurate chromosome segregation by monitoring kinetochore–microtubule attachment and preventing premature anaphase onset. Recent studies have elucidated the molecular mechanisms underlying SAC function, including the role of the Mad2 template model, the involvement of Mps1, and the regulation of the MCC. Understanding theseThe spindle assembly checkpoint (SAC) is a critical quality control mechanism that ensures accurate chromosome segregation during mitosis and meiosis. It delays cell division until all chromosomes are correctly attached to the spindle microtubules via their kinetochores. When kinetochores are not properly attached, the SAC is activated, blocking cell cycle progression. Once all kinetochores are stably attached, the SAC is inactivated, allowing cell division to proceed. This review summarizes recent advances in understanding how the SAC signal is generated, how it blocks cell cycle progression, and how it is extinguished. Eukaryotic cells face unique challenges in ensuring accurate genome transmission due to the separation of genome replication and segregation. Sister chromatid cohesion, mediated by cohesin, ensures that replicated sister chromatids remain attached until all chromosomes are correctly aligned on the spindle. The SAC ensures that this cohesion is maintained until all chromosomes are properly attached, preventing premature separation. The SAC is a complex network involving proteins such as Mad1, Mad2, Mad3, Bub1, Bub3, and BubR1. These proteins are recruited to kinetochores in a stepwise manner, with Bub1 playing a key role in recruiting downstream SAC components. The Mad2 template model explains how unattached kinetochores generate the SAC signal, with Mad2 adopting two conformations (open and closed) to facilitate the formation of the mitotic checkpoint complex (MCC). The MCC then inhibits the anaphase promoting complex (APC/C), preventing premature anaphase onset. Mps1, a protein kinase, is essential for recruiting the Mad1–C-Mad2 core complex to kinetochores and for promoting the template reaction. The role of Mps1 is further supported by its interaction with the spindle matrix, which facilitates efficient kinetochore loading. The SAC signal is extinguished when microtubules attach to kinetochores, leading to the removal of the MCC and the release of Cdc20, allowing the APC/C to activate. The SAC is also regulated by the interaction of BubR1 with Cdc20, with BubR1 containing two conserved KEN boxes that are essential for SAC function. The APC/C inhibitor is primarily the MCC, which is assembled from the Mad2–Cdc20 complex and the Mad3/BubR1–Bub3 complex. The SAC signal is extinguished through mechanisms such as stripping, which removes the MCC from kinetochores, and through the inactivation of existing inhibitory complexes. In summary, the SAC ensures accurate chromosome segregation by monitoring kinetochore–microtubule attachment and preventing premature anaphase onset. Recent studies have elucidated the molecular mechanisms underlying SAC function, including the role of the Mad2 template model, the involvement of Mps1, and the regulation of the MCC. Understanding these