A buoyancy control device, or BCD, is an inflatable vest that can be orally or mechanically inflated with air in order to regulate your buoyancy in the water. The BCD is a mandatory device for all scuba divers and it provides them surface support for resting and swimming, neutral buoyancy underwater, and less buoyancy for descending. Many different styles of BCD's are available, some of which are very sophisticated and require special training to use. When you shop for a BCD, be sure that it features a pressure release valve. And it should have a comfortable strap or harness that prevents the device from creeping up around the diver's neck. Buy a BCD with controls that can be easily located, distinguished, and operated. A low pressure or manual inflation system is also desirable. Ask a diving instructor or a professional diving retailer for more information about buoyancy control devices.