The High Throughput X-Ray Spectroscopy Mission (XMM) is a `Cornerstone' Project in the ESA long-term Programme for Space Science. The satellite observatory uses three grazing incidence mirror modules coupled to reflection grating spectrometers and X-ray CCD cameras. In order to achieve a large effective area, each XMM mirror module shall consist of 58 Wolter I mirrors which are nested in a coaxial and cofocal configuration. This high packing density requires the production and integration of very thin mirror shells with diameters included between 300 and 700 mm. In 1991-93, a development program was run which aims to demonstrate the feasibility of such mirrors. Demonstration models which integrate mirrors having different sizes were manufactured using CFRP replication and Nickel electroforming technologies. These were X-ray tested. The proposed paper summarizes the activities and the test results obtained during this program.