I will introduce blazars and I will discuss their importance for astrophysics and astroparticle physics. Blazars are the sources dominating the sky at gamma-ray energies. Their non-thermal emission, dominated by the relativistically boosted emission from a relativistic jet, is the ideal tool to study the physical processes acting in the jet. The powerful high-energy beam can also be used to probe the extragalactic background light. In recent years blazars have been also considered as high-energy neutrino and ultra-high energy cosmic rays sources.