This paper discusses the circumstances in which competition authorities can review potentially anti-competitive mergers and acquisitions that have been consummated, i.e. the merging parties have integrated the businesses and started operation as a single entity vis-à-vis customers, suppliers and competitors. It focuses in particular on situations where a transaction was not notified, either because it was not subject to notification requirement or because it was subject to a voluntary notification (and the parties decided not to file it). The paper was prepared as a background note for a discussion held at the OECD in February 2014 on investigations of consummated and non-notifiable mergers.