There is disclosed a system for the installation of one or more shelves in a cabinet. Included in the system are a pair of shelf guide rail assemblies which are adapted to be respectively mounted to the side walls of a cabinet for slidably receiving the shelves. The shelves are each of two-piece construction and adjustable in length. The shelves may be provided with a rearwardly disposed slot for receiving the plumbing when the system is installed in a cabinet into which a sink has been installed. A number of precut filler pieces are provided for the filling the spaces between the top surfaces of the respective sections of the shelves.