The standard model (SM) of particle physics is a hugely successful theory of nature, but it is incomplete. E.g., it cannot explain finite SM neutrino masses or the origin of the primordial baryon asymmetry (BAU). One way to address such problems is to postulate the existence of new but hidden particles. This thesis studies such "hidden sectors" in two ways: 1) An effective theory approach, where electroweak (EW) and GeV scale portal effective theories (PETs) are constructed that couple the SM to a generic light hidden mediator of spin 0, 1⁄2, or 1. The EW scale PETs include all portal operators of dimension d≤5. The GeV scale PETs additionally include all leading order (LO) flavour changing portal operators of dimension d≤6,7. They are used to derive a LO PET chiral perturbation theory Lagrangian that describes hidden sector induced light meson transitions in fixed target experiments like NA62 or SHiP. 2) An investigation of the type-I seesaw model, which couples the SM to n≥2 sterile neutrinos that can generate a BAU via “leptogenesis”. It is shown that thermal and spectator effects can result in a sign-flip and strong relative enhancement of the BAU in high-scale leptogenesis with two hierarchical sterile neutrinos of vanishing initial abundance. Much lighter sterile neutrinos may be detected via lepton number violating (LNV) decays at colliders, but LNV decays could be suppressed relative to lepton number conserving decays for 't Hooft natural parameter choices. It is shown that the corresponding parameter space consists of three regions: (a)with unsuppressed LNV decays, (b)with suppressed LNV decays, (c)with suppressed and unsuppressed LNV decays.