Overview of the contributions of the LHC experiments in INFN GRID for bringing the GRID to production quality

The Italian groups participating to the LHC experiments with INFN funding, have given very substantial contributions in bringing the European Grid Infrastructure to the high level of reliability and efficiency that has finally been reached. This research and development work was performed by the members of the computing groups of the experiments, that coordinated themselves within the INFN GRID project. In this paper we present an overview of the Data Challenges the experiments have performed on the Grid Infrastructure with increasing complexity and involvement of sites and resources and highlight the results achieved in steering the grid middleware and operations to real maturity for use in production. The crucial role played by INFN GRID members in the experiments is also illustrated together with the working of the organisms that have allowed feedback from the computing activity of the LHC experiments to be efficiently incorporated in the grid infrastructure and the main steps and results thus achieved