Observational constraints on massive stars & binaries

Massive stars produce heavy elements, dust, and ionising energy that shape the evolution, spectral appearance, and chemical imprints of the Universe at all scales. Observational surveys revealed that binaries are commonplace among massive stars, giving rise to unique phenomena such as stripped supernovae, stellar mergers, and gravitational-wave sources. Understanding the frequency and resulting products of these interactions is crucial, especially in conditions which mimic those of the younger Universe. In my talk, we will take a tour through key evolutionary stages of massive binaries. I will present recent and current observational constraints which are emerging for these populations, focusing on main sequence stars, binary-interaction products, and black holes at solar and sub-solar metallicities.