Abstract: Relativistic cosmic rays (CRs) may play an important role in the evolution of galaxies and clusters by driving galactic winds and heating diffuse gas. As a result, “CR feedback” has become a key ingredient in galaxy evolution models. However, a fundamental limitation of these models is that the nature of CR feedback is a very strong function of the assumed CR transport. Getting a better handle on the microphysics of CR propagation is therefore essential for understanding the macrophysics of CR feedback in galaxies. In this talk, I will discuss theoretical models of CR propagation and how CR spectra measured at Earth help us constrain their transport physics. I will show that existing theories of CR transport remain uncertain and are generally not in good agreement with local observations, which suggests that there may be a significant theoretical gap in our understanding of CR propagation.