  These notes summarize the lectures about "Cosmic-ray propagation in
extragalactic space and secondary messengers", focusing in particular on the
interactions of cosmic-ray particles with the background photons in the
Universe, including nuclear species heavier than hydrogen, and on the
analytical computation of the expected cosmic-ray fluxes at Earth. The lectures
were held at the Course 208 of the International School of Physics "Enrico
Fermi" on "Foundations of Cosmic-Ray Astrophysics", in Varenna (Como, Italy)
from June 23rd to June 29th, 2022. These notes are complementary to the content
of the lectures held by Pasquale Dario Serpico at the same school.
