Abstract We discuss bounds on and implications of a possible fourth generation of quarks and leptons. We derive lower bounds on the masses of fourth generation quarks from the recent experimental bound m τ' ⩾ 41 GeV, for both nonsupersymmetric and supersymmetric models. We also derive upper bounds on these masses from the requirement of a perturbative unification, and briefly discuss the non-perturbative case. We then show that a fourth generation renders the conventional mechanism of cosmological baryogenesis via Higgs-decay problematic, and tends to result in a large supersymmetry breaking scale in minimal supergravity models. We finally show that the incorporation of a fourth generation in superstring models can only work if the superpotential fulfills certain specific conditions, which appear to be in conflict with cosmological requirements.