Retailing in the 21st century will no doubt be very different from retailing in the 20th century, just as retailing in the 20th century was very different from retailing in the 19th century. To understand and explain current retailing phenomena and retailing-related phenomena, and accurately predict such phenomena, it is neccessary to have a coherent and consistent definition of retailing as well as comprehensive retailing theories based on this definition. This introduction to the special issue argues for such a definition and related comprehensive theories.