National oil companies (NOCs) control over 80 percent of world oil reserves and over 50 percent of gas reserves and hold exclusive rights to exploration and development of oil and gas reserves within their home countries. Because of host government involvement and supervision, NOCs may also act as instruments of state, implementing government foreign and domestic policies such as wealth re-distribution through the provision of subsidised oil products, job creation, and economic development. Such activities can lead to restricted availability of funds for finding and developing reserves for future production and to inefficiencies in current production and distribution. This thesis assesses the geopolitical factors that influence the conduct, strategies and priorities of NOCs and how these may impact on the continuing security of energy supplies to countries which are members of the Organisation for Economic Co-operation and Development (OECD). It will focus on ten NOCs supplying oil to world markets and two which supply gas to the European market. The study will also review the activities and .scope for influence as state instruments of Sovereign Wealth Funds which have been established by states with NOCs, in particular, those which have earned substantial petro-dollar surpluses, during the period of high oil and gas prices of2006-2008. In an age of global interdependence between nations, specific objectives of this thesis are to consider the implications of anticipated growth in world demand for oil and gas supplies over the next 20 years, whether world production capacity is likely to grow to meet increases in world demand, the potential impact on world oil and gas supplies of the policies and practices of NOCs, in particular, the desire of host governments to require NOCs to follow non- commercial objectives, and the responses from OECD countries to threats to their energy security from potential restrictions on supplies.