The Fischer-Tropsch (FT) synthesis is the catalytic conversion of hydrogen and carbon monoxide into higher hydrocarbons. Although ruthenium is the most active catalyst in FT synthesis, the lower cost metals iron and cobalt are used commercially. Generally, increased catalyst performance is achieved with increased metal surface area and dispersion, though recent literature reports that cobalt, iron and rhodium crystallites below a certain size in the nanometer range display lower metal surface area specific activity and a high methane selectivity. There have also been several attempts to investigate the possibility to conduct a homogeneously catalysed FT synthesis, which can be considered as an extreme case of metal dispersion. The aim of this study is to investigate FT activity and selectivity on well-defined nano-sized ruthenium crystallites and supported organometallic clusters with varying nuclearity, and to compare/bridge the obtained findings by subdividing it into a) catalyst preparation and characterisation, b) FT performance of model catalysts and c) theoretical investigations of the feasibility of FT reactions on ruthenium complexes.