The buoyancy control device, or "BCD," is used to compensate for buoyancy shifts that occur as you change depth. The most significant factor in these shifts is your wetsuit, which compresses as you descend and returns to its original thickness as you ascend.

A BCD contains an air bladder that is inflated with air from your cylinder. As you descend, you add air to the BCD to compensate for the loss of your wetsuit's buoyancy. Then, as you ascend, you release air to prevent the expansion of your suit and air in your BCD from causing an uncontrolled or rapid ascent.

You also use the BCD to maintain positive buoyancy while on the surface. This allows you to rest at the surface before and after the dive, and also during surface swims.

The modern BCD also includes a strap for securing your cylinder, and many have pockets for stowing small items.