Hydrogel-coated coils are truly detachable coils with a platinum core covered with hydrogel. The coils are available in 0.018- and 0.035-in systems. These coils have the ability to expand up to four times their size ˜20 minutes after deployment, thus providing a very effective mechanical vascular occlusion effect. The vessel-occlusive effect of these coils is a volume, space-occupying effect, not a thrombotic effect, as seen in fibered coils. Hydrogel-coated coils were originally developed and designed to treat brain aneurysms; however, their use has expanded to peripheral applications. Hydrogel-coated coils have been used in the management of visceral aneurysms, high-flow vascular arteriovenous fistulae, and endoleaks after endovascular thoracic and abdominal aneurysm repair. The purpose of this article is to describe the hydrogel-coated coil system, the mechanism of action, technical details for optimal deployment, and clinical applications.