Complex suites of proteins that are secreted by plants and phytopathogens into the plant apoplast play crucial roles in surveillance, assault, defense, and counter-defense. High-throughput genome-scale strategies are being developed to better understand the nature of these "secretomes" and the identity of pathogen-derived effector proteins that subvert plant defenses and promote pathogenicity. Although combined bioinformatic and experimental approaches recently have provided comprehensive coverage of secreted proteins from bacterial phytopathogens, far less is known about the secretomes and batteries of effectors of eukaryotic phytopathogens; notably fungi and oomycetes. The yeast secretion trap (YST) represents a potentially valuable technique to simultaneously target pathogen and host secretomes in infected plant material. A YST screen, using a new vector system, was applied to study the interaction between tomato (Solanum lycopersicum) and the oomycete Phytophthora infestans, revealing sets of genes encoding secreted proteins from both pathogen and host. Most of those from the oomycete had no identifiable function and were detectable in planta only during pathogenesis, underlining the value of YST as a tool to identify new candidate effectors and pathogenicity factors. In addition, the majority of the P. infestans proteins had homologs in the genomes of the related oomycetes R. sojae and P. ramorum.