We have performed ab initio pseudopotential calculations in order to investigate the atomic and electronic structure of pyridine adsorbed on the Ge(100) surface. A large number of pyridine/Ge(100) adsorption configurations possibly resulting from cycloadditions and Lewis acid-base reactions are presented. The configuration having the Ge-N linkage formed by dative bonding with adsorbed pyridine molecules tilted is the most stable, which explains the experimental STM images well. The dative bonding character is investigated by comparing the charge densities for the clean and pyridine-adsorbed Ge(100) surfaces. Finally the difference between the Ge(100) and Si(100) surfaces is discussed.