  To answer the fundamental questions concerning the origin and nature of
ultra-high energy cosmic rays (UHECRs), it is important to confront data with
simulated astrophysical scenarios. These scenarios should include detailed
information on particle interactions and astrophysical environments. To achieve
this goal one should make use of computational tools to simulate the
propagation of these particles. For this reason the CRPropa framework was
developed. It allows the propagation of UHECRs with energies $\gtrsim$10$^{17}$
eV and secondary gamma rays and neutrinos. The newest version, CRPropa 3,
reflects an efficient redesign of the code as well as several new features such
as time dependent propagation in three dimensions, galactic magnetic field
effects and improved treatment of interactions, among other enhancements.
