Ustilago maydis induces tumors in maize. It is dimorphic, with a unicellular, nonpathogenic form, and a dikaryotic, filamentous, pathogenic form that requires the plant for its growth. The life cycle is regulated by two mating type loci, a and b: a has two alleles, which encode a pheromone and a receptor; b is multiallelic and encodes a combinatorial homeodomain protein. Cell fusion occurs between haploid cells with different a alleles. The dikaryon formed exhibits filamentous growth if the cells also carry any two different b alleles. The b locus is the major pathogenicity determinant, fuz7, which encodes a MAP kinase activator, is also necessary for tumor induction and may respond to plant signals. Several other genes have been identified (fuz1, fuz2, rtf1, uac, ubc) that affect different aspects of the life cycle transitions. Some of them may identify targets of the b locus. Signaling between pathogen and plant is of key importance in tumor induction and fungal growth and differentiation within the plant. The role of siderophores is also discussed.