Large-scale genome sequencing projects have generated a wealth of sequence information for plant pathogenic microbes such as oomycetes and fungi. Functional genomic approaches are essential to exploit the sequence information to identify pathogen effector genes that trigger cellular and molecular responses in plant cells. This chapter describes two functional assays, agroinfiltration and agroinfection. These assays allow rapid functional expression of pathogen genes in plants and can be used in high-throughput screens.