The oomycete plant pathogen Phytophthora infestans causes late blight a devastating disease of potato and tomato. P. infestans secretes a diverse array of effector proteins that reprogram host processes to facilitate colonization and pathogenicity. Recent work indicated that P. infestans secretes two classes of effectors that target distinct sites in the host plant. Some effectors are secreted into the plant extracellular space (apoplastic effectors), whereas others are translocated inside the plant cell where they target distinct subcellular compartments (cytoplasmic effectors). We used data mining of genome sequences and functional genomics to identify more than 200 genes that are likely to encode secreted effectors of both classes. Apoplastic effectors are often small cysteine-rich proteins, such as the serine and cysteine protease inhibitors that target host proteases. In contrast, cytoplasmic effectors belong to two distinct and highly diverse families: (1) the RXLR family, and (2) The CRINKLER family. Each of these families is represented by more than 50 different proteins that are characterized by a highly conserved sequence motif at their N-terminus. These motifs are thought to allow translocation into host cells and are similar to host translocation signals reported in malaria parasites. Our goal is to understand the molecular function and biochemical activity of P. infestans effectors. How do they operate to reprogram host defenses? What are their host substrates? How did they coevolve with host proteins? In this presentation, I will review recent findings that address these questions.