The WD-repeat family of proteins is a large family of proteins whose members are structurally homologous but functionally diverse. The WD-repeat motif is defined by four or more repeating units of a conserved core of 40-60 amino acids that begins with a glycine-histidine (GH) dipeptide and ends with a tryptophan-aspartic acid (WD) dipeptide. Regions within the WD-repeat exhibit sequence heterogeneity and are likely responsible for imparting their functional diversity. WD-repeat-containing proteins perform in a spectrum of cellular activities which includes RNA processing, signal transduction, transcription, vesicular trafficking, cytoskeletal assembly, chromatin assembly, and various aspects of cell division. The crystal structure of the beta subunit of the heterotrimeric G-protein has been resolved and has provided a structural characterization of the WD-repeat. Each of the seven repeats of the G-protein beta subunit folds into small antiparallel beta-sheets that radiate from a central pseudo-symmetrical axis to form a beta propeller structure. The surfaces formed by the beta propeller structure are proposed to function as a scaffold capable of coordinating reversible protein-protein interactions. Protein-protein interactions provide a source of complexity for the multitude of physiological processes that must be executed by eukaryotes for development and for the maintenance of homeostasis. Through their conserved beta propeller structure, WD-repeat-containing proteins contribute to this source of complexity and have thus established themselves as a critical subject of study.