The use of synthesis gas offers the opportunity to furnish a broad range of environmentally clean fuels and high value chemicals. However, synthesis gas manufacturing systems based on natural gas are capital intensive, and hence, there is great interest in technologies for cost effective synthesis gas production. Direct production of synthesis gas with flexible H-2/CO ratio, which is in agreement with the stoichiometric ratios required by major synthesis gas based petrochemicals, can decrease the capital investment as well as the operating cost. Although CO2 reforming and catalytic partial oxidation can directly produce desirable H-2/CO synthesis gas, they are complicated and continued studies are necessary. In fact, direct production of flexible H-2/CO synthesis gas can be obtained by optimizing the process schemes based on steam reforming and autothermal reforming as well as partial oxidation. This paper reviews the state of the art of the technologies. (c) 2005 Elsevier Ltd. All rights reserved.