The availability of genome sequences for some of the most devastating eukaryotic plant pathogens has led a revolution in our understanding of how these parasites cause disease, and how their hosts respond to invasion. One of the most significant discoveries from the genome sequences of plant pathogenic oomycetes is the plethora of putative translocated effector proteins these organisms encode. Many effector genes display signatures of rapid evolution and tend to reside in dynamic regions of the pathogen genomes. Once inside the host, effector proteins modulate cellular processes, mainly suppressing plant immunity –. Effectors can also be recognized directly or indirectly by the plant immune system through the action of disease resistance (R) proteins.