H ere we explore the phenomenological and mechanistic bases o f dominance. Dominance can be defined as an operational measure o f the deviation o f the heterozygotes observed trait value from its expected value based on the phenotype o f the two homozygotes. A simple mathematical analysis o f various measures o f dominance shows that its main source is the existence o f nonlinear relationships between the genotypic and phenotypic values. How= ever, the mechanisms generating these nonlinearities are diverse. We will oudine the molecular mechanisms o f various cases o f dominant phenotypes. As dosage imbalances lead to an impor= tant class o f dominant mutations we attempt to explain these effects from a theoretical perspec= tive. Finally, we discuss issues concerning robustness against deleterious mutations from the perspective o f gene redundancy, the topology o f cellular networks and other buffering effects to maintain a normal phenotype .