Following the recent publication of a comprehensive dataset of 2400 genes with a loss-of-function mutant phenotype in Arabidopsis (Arabidopsis thaliana), questions remain concerning the diversity of dominant mutations in Arabidopsis. Most of these dominant phenotypes are expected to result from inappropriate gene expression, novel protein function, or disrupted protein complexes. This review highlights the major classes of dominant mutations observed in model organisms and presents a collection of 200 Arabidopsis genes associated with a dominant or semidominant phenotype. Emphasis is placed on mutants identified through forward genetic screens of mutagenized or activation-tagged populations. These datasets illustrate the variety of genetic changes and protein functions that underlie dominance in Arabidopsis and may ultimately contribute to phenotypic variation in flowering plants.