Publisher Summary This chapter discusses various assays of cell and nuclear fusion. The pathway of yeast mating forms a microcosm of the cell biological universe, encompassing fundamental problems in signaling, transcriptional regulation, polarization, motility, and membrane fusion. Each step in the pathway has been defined and dissected by the isolation and analysis of numerous mutations that cause specific defects in the efficiency of mating. Certain profoundly mating defective mutants may reduce the efficiency of mating by more than five orders of magnitude. Such mutants are generally called “sterile” or “ste” and are caused by defects in the initial pheromone signaling or the subsequent response pathways. In general, there are two basic ways to detect defects in cell and nuclear fusion. The first approach uses sensitive methods of detecting either the reduced efficiency of diploid formation or—in the case of nuclear fusion—the increased production of unique progeny cells called “cytoductants.” The second approach uses microscopic methods to detect the distinct zygotes in which cell or nuclear fusion has failed. It has the virtue of providing information about the nature of the mating defect but—because of the relatively fewer numbers of cells examined—may provide quantitatively less precise data.