A key feature of WLCG's multi-tier model is a robust and reliable file transfer service that efficiently moves bulk data sets between the various tiers, corresponding to the different stages of production and user analysis. We describe in detail the file transfer service both the tier-0 data export and the inter-tier data transfers, discussing the transition and lessons learned in moving from a reliable software product, the gLite FTS, to a full production service based on that software. The focus is upon the deployment and operational experience of the service gained during the 2006 and 2007 experiment production activities and dress rehearsals. We discuss the software and operational features that have been deployed to meet the reliability and performance needs of the service, and integration of the service with the WLCG and experiment operations.