An apparatus for testing electronic devices, having a test head coupled to at least one immovably mounted test socket, a positioning device for positioning the electronic device in testing position and a lead-backer attached to the positioning device for supporting the electronic device and pressing it against the test socket. A supply port for supplying a temperature control medium to a temperature control system of the said lead-backer is immovably mounted beside the said test socket, the said temperature control system of the said lead-backer and the said supply port communicate with each other when the electronic device is in testing position, whereby the said temperature control medium flows from the said supply port to the said temperature control system of the said lead-backer.