Detecting gamma-rays from discrete astronomical objects offers the potential to directly study sites of cosmic particle acceleration. Alternatively, the pervasive diffuse gamma-ray emission component caused by interactions of cosmic rays with interstellar gas and radiation fields carries information about the spatial distribution of cosmic particle accelerators and reveals details about the character of cosmic ray propagation in the Galaxy. Cosmic ray interactions leaving observable gamma-ray phenomena can be studied already in our solar system, but reach out to the Large Magellanic Cloud and the nearest most luminous starburst galaxies. Observations with both space- and ground-based gamma-ray telescopes complement each other when studying the signatures of cosmic ray interaction, acceleration, and propagation. Show Partial Abstract