Dominance is a ubiquitous phenomenon of great evolutionary importance. We review empirical facts about the degree of dominance, present theories explaining its molecular basis, outline empirical facts and theories on the evolutionary modification of dominance, and discuss the controversies revolving around this topic. We conclude that molecular pathways can often generate dominance due to inherent physiological constraints, and that the latter do not necessarily require evolutionary explanations based on selection. Nonetheless, molecular pathways can allow for dominance modification, and hence, dominance evolution. Furthermore, evolution of dominance has been demonstrated in many well-studied ecological settings, and whether due to selection or otherwise, their evolutionary origin needs to be addressed.