This chapter explains epistasis, whereby the phenotype of one gene masks the phenotype of a different gene, and describes how it can be exploited to construct the logical pathway of the gene interactions that underlie a biological process. It notes that every biological process is the outcome of genes or gene products working together in pathways and networks, which may be linear or branched, and may involve many genes or few genes. The chapter discusses how gene interactions in negative, positive, and parallel pathways are inferred by using double mutants.