Ammonia is a good model system for the study of co-adsorption interactions, including indirect effects such as charge and strain-induced local effects on adsorption sites, and direct interactions such as hydrogen bonding. On the Si(001) surface, it adsorbs molecularly, via a dative bond from the N atom to the down atom of a buckled dimer, and is therefore very sensitive to the local charge conditions. It will then dissociate into–H and–NH2 groups, adsorbed on the dangling bonds of the Si dimers. The NH2 groups do not diffuse, so any correlations deriving from interactions during adsorption are preserved, and can be derived by analysis of the arrangements of the NH2 groups. Hydrogen-bonding interactions are crucial in understanding the behaviour of this system, with significant co-adsorption interactions occurring both along and across rows, outweighing the electrostatic or buckling-related effects. In recent years, there have been several scanning tunnelling microscopy studies and extensive computational modelling of the NH3 on Si(001) system, attempting to determine a dominant mechanism governing co-adsorption effects. In this review, I will discuss both experimental and theoretical results, make a comparison with similar molecules such as phosphine (PH3), and review the different ways in which experimentalists and modellers have approached this complex system.