First-Principles DFT methods can provide useful information regarding detailed reaction mechanisms. In this talk, we will present our recent work on the early steps of Fischer-Tropsch synthesis on iron and cobalt based catalysts. A number of elementary reaction steps leading to CO dissociation and hydrogenation of carbon-containing species have been analyzed both in terms of their thermochemistry and minimum energy paths. A comparison between DFT results and experimental results on realistic FTS catalysts leads to some interesting conclusions, including the role of hydrogen in CO dissociation on Fe and Co catalysts, and the possible role of identified surface reaction intermediates in the FTS polymerization reaction.