The chapter focuses on the switching and mating processes of Candida species. Hull and Johnson found orthologs of three Saccharomyces cerevisiae mating-type genes in the Candida albicans mating type-like (MTL) locus. Although the scheme for mating and the regulation of the response to the mating pheromones proved in subsequent studies to be in general similar to that of S. cerevisiae and hence that of the hemiascomycetes in general, the biggest surprise was the fundamental and unique role that white-opaque switching played in the mating process of C. albicans. The response of an opaque cell to pheromone produced by the opposite mating type was shown to be highly similar to that of S. cerevisiae, including the components of the major regulatory pathway that transduces the pheromone signal. The spontaneous transition between the white and opaque phenotypes by a selected group of strains had been shown to affect a number of phenotypic and virulence traits before the discovery by Miller and Johnson that it was an essential step in the mating process. The sensitivity of the opaque phenotype to high temperature caused a conundrum regarding the relationship of host and mating. The host signals that have been identified stimulate the rate of white-to-opaque switching, but switching occurs spontaneously and in both directions. Gradients of pheromone would be extremely prone to mechanical disruption and dissipation by diffusion if in a purely liquid environment. A biofilm would provide a protective environment against disruption, and a porous matrix would reduce diffusion.