Observational constraints on massive binaries

Binary interactions are commonplace among massive stars, giving rise observed phenomena such as X-ray binaries, stripped stars & supernovae, and gravitational-wave sources. The multiplicity properties of massive stars thus represent a fundamental observable to calibrate, test, and benchmark models of single-star and binary evolution. In these proceedings, I provide a modern summary of the observed properties of massive binaries across various metallicities, and discuss open problems in the field.