Since the airline sector has been de-regulated, air travel has to a large extent become a commodity and air services have become market-driven. Driven by lower prices and growing income, air traffic for passengers and cargo has grown exponentially. Airlines started offering lower fares to consumers under the pressure from increasing competition, which was spurred by liberalisation and de-regulation of the air transport sector. While many agree that overall the evolution of the sector has produced significant benefits to consumers, concerns about anti-competitive behaviours remain. One of the most striking features of the international air transport sector is the variety and number of co-operation arrangements between airlines. These are typically used to reduce costs and spread the operational risk. However, any kind of horizontal co-operation between airlines, be it a basic interlining agreement or a full horizontal merger, may produce anti-competitive effects. In addition, many airlines (in particular incumbent carriers) may still have the ability and incentives to abuse the dominant position they inherited from the period preceding liberalisation. For example, legacy carriers may use their privileged position in a hub airport or engage in a predatory strategy with a view to deterring entry into the market in which they are present. Given the nature and scope of air transport activities, the lack of a level playing field as well as harm from anti-competitive transactions or behaviours in the airline sector may have a deleterious effect on other sectors as well as the economy as a whole. This Background Note highlights the most pressing and timely concerns that competition authorities are likely to face with respect to the international air transport sector. It focuses on civil international air transportation for passengers. Vertical aspects (e.g. airline-agent relationship) and purely cargo and domestic matters are excluded for purposes of the present Roundtable and Background Note. Section 1 describes the current regulatory framework and main features of the airline industry. Section 2 provides an overview of the competitive landscape in which airlines operate, and Section 3 addresses antitrust enforcement and airline competition issues.