The world demand of light and middle distillate petroleum products (mainly gasoline and diesel) is increasing dramatically, especially within the transportation sector. Eventually, such requirement will surpass refineries capacity to supply fuels and hydrocarbon feedstock. This situation has encouraged the exploration of other sources of petroleum products rather than merely sticking to conventional oil sources. This book describes the active sites and reaction intermediates with Fe-based catalysts in relation to the most relevant aspects of Fischer-Tropsch Synthesis with Co and Ru catalysts.