Roland Diehl obtained a degree in nuclear physics from the Johannes Gutenberg university in Mainz, Germany, in 1978. He obtained his PhD in astrophysics from TU Munich on gamma ray observations of the 26Al radioisotope in our Galaxy, in 1988. He has been teaching at the Munich Technical University since 1998, with a professorship at TU Munich since 2010. Roland Diehl has been staff scientist at the Max Planck Institut fuer extraterrestrische Physik (MPE) in Garching since 1979, as a senior staff member of the MPE's high-energy astrophysics group, from which he was retired in 2019. He continues scientific activities since then as a gues scientist of MPE and other international host institutions. Roland Diehl was elected member of the Senate of the Max Planck Society from 2009 to 2015, and elected member of the German astroparticle physics committee from 2009 to 2022. His science career focused on gamma ray telescope instrumentation in earlier years. As a core member of the COMPTEL instrument team with NASA's Compton Gamma-Ray Space Observatory (1991-2000), and Co-Principal Investigator of the SPI spectrometer instrument on ESA's INTEGRAL space mission (2002-2025), he specialized in gamma-ray line spectroscopy. His science focuses mainly on cosmic nucleosynthesis observations and their interpretation with characteristic gamma-ray lines from radioactive isotopes, but also on spectroscopy from positron annihilation and cosmic-ray interactions. Main interests are the roles of massive stars and their supernovae in cosmic nucleosynthesis, and in the distribution and recycling of matter through the interstellar medium.Astronomy through electromagnetic radiation includes gamma rays at its high-energy end. In this regime beyond thermal processes and of relativistic astrophysics, spectral lines carry unique information about nuclear processes in cosmic environments, and about interactions of relativistic particles such as annihilations with their antiparticles. Characteristic nuclear gamma rays originate from radioactive decays of short-lives isotopes that are by-products of nucleosynthesis reactions in stars and stellar explosions. Annihilation of positrons results in a characteristic gamma-ray spectrum that allows investigations of cosmic sources of positrons. We will discuss how astrophysical questions related to stellar evolution and stellar explosions, and investigations of the interstellar medium with its cosmic relativistic particles, have been stimulated by gamma-ray spectroscopy. We will also address the measurement techniques for cosmic gamma rays and how these could be advanced.