Female breasts are formed by fat and glandular tissue, blood vessels, nerves, milk lobules and ducts and lymph ducts. Fat tissue makes up a large proportion; often the majority of tissue within the breasts. Differences in breast size are due primarily to differences in the amount of fat tissue, as opposed to differences in duct or lobule volume. It is the fat tissue, together with the skin elasticity that makes your breasts feel soft and gives them their shape. Each breast lies over a muscle of the chest called the pectoral muscle. A breast implant can be placed over, under or partially under this muscle.

The breast is a subcutaneous structure. Fat is injected in the subcutaneous layers and not in the breast itself.