The characterization of gene function typically includes a detailed analysis of loss-of-function alleles. In model plants, such as Arabidopsis thaliana and rice, sequence-indexed insertion collections provide a large resource of potential null alleles that can often be easily accessed through convenient Web sites (e.g., http://signal.salk.edu ). They are, however, not available for nonmodel species, require stacking for knockout of redundant homologs, and do not easily allow for partial or regulated loss of gene function, which is particularly useful when null alleles are lethal. Transgene approaches that employ directed gene silencing can substitute for null alleles and also enable refined studies of gene function, e.g., by tissue-specific and inducible gene-silencing. This chapter describes the generation and application of artificial microRNAs (amiRNAs) as a gene silencing tool in a wide variety of different plant species.