Activation tagging is a mutagenesis strategy that generates dominant, gain-of-function mutations as a consequence of gene over-expression. These mutations cause a class of mutant previously unobtainable by conventional mutagenesis. Unlike most mutant phenotypes, which are generally a consequence of gene inactivation, activation tagged phenotypes arise from excess functional gene product. Gene over-expression mutations are obtained by randomly inserting regulatory sequences throughout the genome, using either high-throughput plant transformation or mobile transposable elements to distribute these regulatory elements. Since the sequence of the regulatory element vector is known, it acts as a molecular tag, making isolation of the over-expressed gene a relatively straightforward process using standard molecular biological techniques. Activation tagged phenotypes have been generated by the over-expression of genes encoding a diverse range of protein and RNA products that are involved in all aspects of plant biogenesis. This mutation approach has been used extensively in Arabidopsis and to a lesser extent in several other species. In this review we summarise activation tagging in plants and suggest that the development of this mutagenesis strategy in more plants of agronomic significance is highly desirable.