  Galaxy evolution is an important topic, and our physical understanding must
be complete to establish a correct picture. This includes a thorough treatment
of feedback. The effects of thermal-mechanical and radiative feedback have been
widely considered, however cosmic rays (CRs) are also powerful energy carriers
in galactic ecosystems. Resolving the capability of CRs to operate as a
feedback agent is therefore essential to advance our understanding of the
processes regulating galaxies. The effects of CRs are yet to be fully
understood, and their complex multi-channel feedback mechanisms operating
across the hierarchy of galaxy structures pose a significant technical
challenge. This review examines the role of CRs in galaxies, from the scale of
molecular clouds to the circum-galactic medium. An overview of their
interaction processes, their implications for galaxy evolution, and their
observable signatures is provided and their capability to modify the thermal
and hydrodynamic configuration of galactic ecosystems is discussed. We present
recent advancements in our understanding of CR processes and interpretation of
their signatures, and highlight where technical challenges and unresolved
questions persist. We discuss how these may be addressed with upcoming
opportunities.
