During infection, plant pathogens secrete effector proteins that target host cellular processes to promote disease. Phytopathogenic fungi possess highly diverse effector repertoires but the precise function of these effectors is largely unknown. Our group has identified a large fungal effector family, called MAX (Magnaporthe AVRs and ToxB-like). MAX effectors share a conserved 3D structure but their sequences and surface properties are highly diversified, suggesting that they target a broad range of host proteins. In the blast fungus Magnaporthe oryzae, MAX effector genes are particularly numerous. Indeed, we identified ~60 MAX genes per genome in a set of 121 isolates of M. oryzae, representing nine host-specific lineages. More than half of these MAX genes are specifically expressed during the biotrophic phase of rice infection in a host genotype dependent manner. We also show that MAX genes represent a highly dynamic compartment of the M. oryzae genome. Pangenome analyses revealed extensive absence/presence polymorphisms and identified MAX gene losses that are associated with host range expansion. My work indicates that such lineage specific losses in MAX genes did not primarily occur to escape immune detection in the new host, but might be explained by other evolutionary forces. In addition, I generated knockout mutants in MAX genes and used TurboID/MS to identify MAX effector targets in rice, providing new insight into the role of these effectors in fungal virulence.