Abstract: Data Grids are currently proposed solutions to large scale data management problems including efficient file transfer and replication. Large amounts of data and the world-wide distribution of data stores contribute to the complexity of the data management challenge. Recent architecture proposals and prototypes deal with replication of read-only files but do not address the replica synchronisation problem. We propose a new Grid service, called Grid Consistency Service (GCS), that sits on top of existing Data Grid services and allows for replica update synchronisation and consistency maintenance. We give models for different levels of consistency provided to the Grid user and discuss how they can be included into a replica consistency service for a Data Grid.