Finding a compatible mating partner is an essential step in the life cycle of most sexually reproducing organisms. Fungi have two or more mating types, and only cells of different mating type combine to produce diploid cells. In mushrooms, this is taken to extremes, with the occurrence of many thousands of mating types. But, having gone to such extraordinary lengths to ensure that almost any two mushroom mycelia in the wild can mate, cell fusion is not followed by nuclear fusion and true diploidy. Instead, the fused cells form a characteristic mycelium, known as the dikaryon, in which haploid nuclei are paired but actively prevented from fusing. The mating-type genes, which encode pheromones, pheromone receptors and homeodomain transcription factors, have crucial roles in regulating the complex developmental programme by which the dikaryon is formed.