Cassiopeia A is a famous supernova remnant, the product of a gigantic explosion of a massive star about 350 years ago. Although discovered in radio observations 50 years ago, we now know that its emitted radiation spans from radio through high-energy gamma rays. It is also one of the few remnants for which the birth date and the type of supernova are known. It was a type IIb, the result of a core collapse supernova explosion. The precise knowledge of its nature makes Cassiopeia A one of the most interesting and investigated objects in the sky, and in particular, the study of its connection with cosmic rays, subatomic particles that fill the galaxy with energies higher than anything achievable in laboratories on Earth.