Annotation of plant genomic sequences can be separated into structural and functional annotation. Structural annotation is the foundation of all genomics as without accurate gene models understanding gene function or evolution of genes across taxa can be impeded. Structural annotation is dependent on sensitive, specific computational programs and deep experimental evidence to identify gene features within genomic DNA. Functional annotation is highly dependent on sequence similarity to other known genes or proteins as the majority of initial "first-pass" functional annotation on a genomic scale is transitive. Coupling structural and functional annotation across genomes in a comparative manner promotes more accurate annotation as well as an understanding of gene and genome evolution. With the increasing availability of plant genome sequence data, the value of comparative annotation will increase. As with any new field, methodologies are evolving for genome annotation and will improve in the future.