One of the most important parameters of a hearing aid is its gain characteristics. Under ideal circumstances, the gain and the functional gain(FG) are the same for an air conduction device. This is not the case, however, with bone conduction devices, e.g., the bone-anchored hearing aid(BAHA). In this article, the relation between the gain and the FG is derived for bone conduction devices including the BAHA. Reference quantities used when measuring bone conduction devices also are presented.