The Intra-Cluster Plasma constitutes an archive of the past history of all clusters and of many groups of galaxies: statistical observations of their X-ray emission will set significant constraints on the dynamical, thermal and chemical events in these cosmic structures. Data on the local X-ray luminosity function N(L, z ≃ 0) were provided by the 1st generation X-ray surveys. HEAO II provided an integral of N(L, z) out to z ≃ 0.4, the counts from a subsample of the MSS (Gioia et al. 1984, Ap.J. 283, 495): these counts result very flat, cf. Fig. 2.