Governments are significant purchasers of goods and services and these markets represent huge opportunities for international trade. This document presents a synthesis of the analytical studies on the quantification of the size of government procurement markets and proposes a series of estimates of procurement markets for over 130 individual countries. The world total government procurement is estimated to amount to $5550 billion in 1998, which is equivalent to 82.3% of the world merchandise and commercial services exports in 1998. The world value of government procurement that is potentially opened up to international trade is estimated at $2083 billion, which is equivalent to 7.1% of the world Gross Domestic Product (GDP) or 30.1% of the world merchandise and commercial services exports. Detailed estimates of government procurement markets are proposed for OECD countries, including estimates of procurement broken down by government levels, types of expenditure (consumption versus investment) and contestable shares. Informed knowledge about the size of government procurement markets and in particular the shares of national markets that are potentially opened up to intenational competition is relevant for the business community, government and trade negotiators.