A Tool Flow System, TFS, is an integral part of a wider system within a Flexible Manufilcturing System, FMS: the Material Flow System. This also includes the Work Flow System.Tool Flow Systems deal essentially with the storage, transport and handling of tools which are exchanged at the machine spindles. Work Flow Systems are particularly concerned with the storage, transport and handling of parts and auxiliary handling devices, such as fixtures and pallets.This article focuses attention on the building blocks ofTFS's and presents a classification, discussion of application and advantages of tool flow fundamental structures. It is argued that the configuration of TFS's and the level of tool replication, may greatly influence the performance and fleXIbility of production in FMS's. Keywords Flexible manufBcturing systems, FMS, tool flow systems, multiple stage systems, MS, combined stage systems, CS and single stage systems, SS. L. M. Camarinha-Matos (ed.), Re-engineering for Sustainable Industrial Production