The book provides an overview of the advantages in using lead enriched with stable lead isotope lead-208 instead of natural lead as a coolant of fast reactors (FRs) and accelerator driven systems (ADS). Lead-208 as a twice magic nucleus has low cross sections of neutron radiation capture and small moderation of neutrons in the range of neutron energies 10 eV – 20 MeV, i.e. in FR and ADS core neutron spectra. These unique features of lead-208 lead to the economy of neutrons, hardening the neutron spectra and other profitable factors.