  The scenario of baryogenesis through leptogenesis is reviewed in models
involving heavy Majorana neutrinos. The various mechanisms of CP violation
occurring in the out-of-equilibrium lepton-number-violating decays of heavy
Majorana neutrinos are studied within a resummation approach to
unstable-particle mixing. It is explicitly demonstrated how the resummation
approach preserves crucial field-theoretic properties such as unitarity and CPT
invariance. Predictions of representative scenarios are presented after solving
numerically the Boltzmann equations describing the thermodynamic evolution of
the Universe. The phenomenological consequences of loop effects of heavy
Majorana neutrinos on low-energy observables, such as lepton-flavour and/or
lepton-number non-conservation in $\tau$ and $Z$-boson decays and electron
electric dipole moment, are discussed.
