Bardet–Biedl syndrome (BBS: OMIM 209900) is a rare developmental disorder that exhibits significant clinical and genetic heterogeneity. Although modeled initially as a purely recessive trait, recent data have unmasked an oligogenic mode of disease transmission, in which mutations at different BBS loci can interact genetically in some families to cause and/or modify the phenotype. Here, I will review and discuss recent advances in elucidating both genetic and cellular aspects of this phenotype and their potential application in understanding the genetic basis of phenotypic variability and oligogenic inheritance.