Heavy neutral fermions constitute a very minimal extension of the Standard Model and can explain a variety of observations, from neutrino masses to the baryon asymmetry to dark matter. In absence of a symmetry which distinguishes them from their Standard Model counterparts, they mix with active neutrinos. Their mass is unconstrained from a theoretical and experimental point of view, with preference for very heavy masses in GUT models or very light if a naturalness principle is advocated. In this talk, I will review the theoretical, experimental and cosmological implications of heavy neutral fermions from the eV to the GUT scale, focusing on the origin of neutrino masses and the baryon asymmetry. Given recent interest in sub-GeV new physics, I will also comment on the current experimental situation, discussing the hints for their existence (including very recent developments) and their tests in neutrino oscillation, beta decay, beam dump, SBN, DUNE, SHIPs and other experiments.