Source: Web Sites of Port Holdings.
(Detailed PDF map)
Container Terminals of the World's Four Major Port Holdings, 2010
Four major port holdings have substantial global assets of about 45 dedicated port terminals each; APM Terminals (controlled by the Danish maritime shipper Maersk), Dubai Ports World (DPW), Hutchison Port Holdings (Hong Kong), and the Port of Singapore Authority (PSA). Jointly, they controlled through various equity stakes 177 dedicated maritime container terminals in 2010. Their assets are geographically diversified, with Pacific Asia being the main focus of HPH and PSA, South Asia and the Middle East having DPW well represented and APM having a portfolio with a strong North American emphasis. They are particularly focused along the world's main commercial gateways, such as the Pearl River Delta (Hong Kong) and the Rhine / Scheldt Delta (Rotterdam and Antwerp). 
Several other port holdings exist, owned by specialized private companies (such as SSA for North America or Eurogate for Europe), by ocean carriers (Hanjin and Evergreen have notable assets) or by financial holdings (Ports America owned by AIG), but their focus is mostly regional. A concentration of ownership among four major port holdings is taking place. In 2006 when DPW acquired the terminal assets of P&O (Peninsular & Oriental Ports) further consolidating its global holdings. However, DPW was constrained to rescind the American assets of this transaction (terminals in Baltimore, Miami, New Orleans, New York and Philadelphia) to the holding AIG (Ports America) due to a political controversy; a Middle Eastern holding operating major American port terminals was perceived negatively in the post 9-11 setting.
Port holdings are thus mainly the outcome of horizontal integration through expansion and mergers with the process leading to a high level of concentration of the global containerized throughput. The seven major port holdings handled a throughput of about 172 million TEUs in 2009, which amounts to 33% of the global throughput.