The chapter discusses the implications of a possible heavy $Z'$ gauge boson associated with an additional $U(1)'$ symmetry in particle physics and cosmology. It covers motivations for such particles, their masses and couplings, and anomaly-free models. The chapter surveys current limits from electroweak and collider experiments and prospects for discovery at future colliders. It explores various theoretical implications, including extended Higgs and neutralino sectors, exotic fermions, flavor-changing neutral currents, neutrino mass, supersymmetry breaking, and cosmological implications for cold dark matter and electroweak baryogenesis. The chapter also delves into specific models, such as those inspired by grand unified theories and string theories, and discusses the challenges and opportunities presented by the existence of a $Z'$ boson.The chapter discusses the implications of a possible heavy $Z'$ gauge boson associated with an additional $U(1)'$ symmetry in particle physics and cosmology. It covers motivations for such particles, their masses and couplings, and anomaly-free models. The chapter surveys current limits from electroweak and collider experiments and prospects for discovery at future colliders. It explores various theoretical implications, including extended Higgs and neutralino sectors, exotic fermions, flavor-changing neutral currents, neutrino mass, supersymmetry breaking, and cosmological implications for cold dark matter and electroweak baryogenesis. The chapter also delves into specific models, such as those inspired by grand unified theories and string theories, and discusses the challenges and opportunities presented by the existence of a $Z'$ boson.