**2.5 Concluding remarks**

The WLCG integrates and operates resources distributed all over the world and its task is to make all these resources accessible and usable for the LHC experiments to distribute, archive and process the data produced by the LHC. This task is done using a specialized software called "middleware" because it sits between the operating systems of the computers at the WLCG sites and the Physics applications software used for the reconstruction, analysis and simulation of the LHC data (or any other application software layer). The middleware is a collection of protocols, agents and programs/services which we describe in the following section.
