A system comprising a heat sink and an electronic unit , the heat sink having a body which includes a channel-shaped mounting formation configured to hold the electronic unit captive therein, the channel-shaped mounting formation defining a mouth having a transverse width which is less than a width of the electronic unit, and a nook which is configured to permit mounting of the electronic unit to the heat sink by passing the electronic unit through the mouth and angularly displacing the electronic unit relative to the mounting formation of the body, wherein the body is elongate and configured for heat exchange with the electronic unit and wherein the channel-shaped mounting formation defines a slot extending along a longitudinal axis (A) of the body, the slot being configured to receive the electronic unit by relative angular and transverse displacement of the electronic unit and the mounting formation of the body, the mounting formation further being configured removably to hold the electronic unit captive in the slot, wherein the slot includes the mouth and an inner receiving region which includes the nook which is disposed toward one side, wherein the inner receiving region has a transverse width which is greater than the width of the electronic unit, the inner receiving region being configured to permit the electronic unit to be received therein via the mouth by relative angular and transverse displacement of the electronic unit and the body, wherein the channel-shaped mounting formation is configured such that the electronic unit is receivable in the nook of the inner receiving region via the mouth by: angling the electronic unit relative to the mounting formation such that the electronic unit is inclined relative to an interface surface of the mounting formation; inserting an insertion side of the electronic unit into the nook of the inner receiving region; angularly displacing the electronic unit relative to the mounting formation about the longitudinal axis until the electronic unit is contiguous with the interface surface of the mounting formation; and transversely displacing the electronic unit relative to the mounting formation until the electronic unit is held captive in the inner receiving region in a configuration in which a contact surface of the electronic unit is continuously contiguous with the interface surface of the mounting formation from a non-insertion side of the electronic unit to the nook in order to ensure maximum heat dissipation from the electronic unit to the body of the heat sink, characterized in that the interface surface is generally flat or planar, but includes a transversely inclined portion which defines the nook in a side of the inner receiving region . angling the electronic unit relative to the mounting formation such that the electronic unit is inclined relative to an interface surface of the mounting formation; inserting an insertion side of the electronic unit into the nook of the inner receiving region; angularly displacing the electronic unit relative to the mounting formation about the longitudinal axis until the electronic unit is contiguous with the interface surface of the mounting formation; and transversely displacing the electronic unit relative to the mounting formation until the electronic unit is held captive in the inner receiving region in a configuration in which a contact surface of the electronic unit is continuously contiguous with the interface surface of the mounting formation from a non-insertion side of the electronic unit to the nook in order to ensure maximum heat dissipation from the electronic unit to the body of the heat sink, characterized in that the interface surface is generally flat or planar, but includes a transversely inclined portion which defines the nook in a side of the inner receiving region . <EOS>