Today ethylene oxide can be produced industrially with ~90 % selectivity through the epoxidation of ethylene over silver catalyst. The past decades there has been a substantial increase in the understanding of the molecular chemistry that leads to high selectivity catalysis. Especially the discovery of an oxometallacycle intermediate that produces ethylene epoxide in competition with acetaldehyde can be considered a major advancement. The state of the surface at reaction conditions causes different reaction paths to compete. At high oxygen coverage also a direct epoxidation channel opens. We will also review recent progress on the understanding of promotion and coverage dependent reactivity. The contributions in our understanding of this reaction from computational catalysis will be emphasized.