<p>Scour around bridge piers continues to pose a critical challenge in hydraulic engineering, threatening the structural integrity and safety of bridge infrastructure worldwide. Among the various countermeasures developed, collar systems have gained prominence for their effectiveness in mitigating local scour by altering near-bed flow structures and attenuating the strength of horseshoe vortices at the pier base. This review synthesizes state-of-the-art research on collar systems, with a focus on their underlying hydrodynamic mechanisms, performance efficacy under diverse flow conditions, and key design parameters that influence their effectiveness. Both empirical and numerical studies indicate that collars, particularly circular types installed at or slightly below the bed level, can significantly reduce local scour depth. Moreover, multi-collar configurations and hybrid systems combining collars with other countermeasures have shown enhanced performance in mitigating scour around bridge piers. Despite their demonstrated effectiveness, the practical implementation of collar systems faces several challenges, including accurate placement under complex hydraulic conditions, vulnerability to debris accumulation, and the need for regular inspection and maintenance. This review systematically evaluates these limitations while also emphasizing the advantages of collar systems as cost-effective, low-maintenance alternatives to conventional countermeasures. The paper concludes by identifying key research gaps and outlining future directions.</p>