The η and η' mesons are almost unique in the particle universe since they are Goldstone boson and the dynamics of their decay are strongly constrained. Because the eta has no charge, decays that violate conservation laws can occur without interfering with a corresponding current. The integrated eta meson samples collected in earlier experiments have been about 10^9 events, dominated by the WASA at Cosy experiment, limiting considerably the search for such rare decays. A new experiment, REDTOP, is being proposed, either at the proton Delivery Ring of Fermilab or at the PS at CERN, with the intent of collecting more than 10^13 η/year (10^11 η'/year) for studying of rare η decays. Such statistics are sufficient for investigating several symmetry violations, and for searches of new particles beyond the Standard Model. Recent studies have indicated that REDTOP has vey good sensitivity to processes that couple to the Standard Model through three so-called portals: the vector, the scalar and the axion portal. An upgraded version of the detector will be proposed, at a later stage, for running at PIP-II, where the η mesons will be produced with tagging. The physics program, the accelerator systems and the detector for REDTOP will be discussed during the presentation.