The η and η mesons are nearly unique in the particle universe since they are almost Goldstone bosons and the dynamics of their decays are strongly constrained. The integrated η-meson samples collected in earlier experiments amount to ∼ 10 9 events. A new experiment, REDTOP (Rare Eta Decays To Observe Physics Beyond the Standard Model), is being proposed, with the intent of collecting a data sample of order 10 14 η (10 12 η ) for studying very rare decays. Such statistics are sufficient for investigating several symmetry violations, and for searching for particles and fields beyond the Standard Model. In this work we present several studies evaluating REDTOP sensitivity to processes that couple the Standard Model to New Physics through all four of the so-called portals: the Vector, the Scalar, the Axion and the Heavy Lepton portal. The sensitivity of the experiment is also adequate for probing several conservation laws, in particular CP , T and Lepton Universality, and for the determination of the η form factors, which is crucial for the interpretation of the recent measurement of muon g − 2.