The Treaty establishing the European Community provides that Members of the European Commission be subject as a body to a vote of approval by the European Parliament prior to their appointment. The practice was therefore established to hold hearings of each Commissioner designate before the responsible parliamentary committees.
As a general rule, in accordance with the Framework Agreement on relations between the European Parliament and the European Commission, hearings have also generally been organised when one or several new Commissioners were to be appointed during the course of a Commission's term.
Please find below links to the websites of hearings of Commissioners designate.