A hearing aid includes a device housing to be worn outside an ear of a user and which co-acts with an in-the-ear part provided with a sound-emitting opening and which is intended and adapted to be received at least substantially in the ear of the user. The in-the-ear part is physically separated from the device housing, wherein at least a microphone and a loudspeaker are accommodated together with the sound-emitting opening in the in-the-ear part. An electronic connection is present between the device housing and the in-the-ear part. A power supply of a hearing aid particularly includes a capacitor (40), more particularly an ultra-capacitor. The functionality of a hearing aid can be expanded with an expansion unit (50). For an accurate fit of the in-the-ear part use is made of a digital representation which has been modified on the basis of fitting data of a fitting body.