With stellar masses in the range of eight to several hundreds of solar masses, massive stars are among the most important cosmic engines. Each individual object strongly impact its local environment, and entire populations of massive stars have been driving the evolution of galaxies throughout the history of the Universe. Over the last two decades, it has become increasingly clear that massive stars do not form nor live in isolation but rather as part of a binary or higher-order multiple system. Understanding the life cycle of massive multiple systems, from their birth to their death as supernovae and long-duration gamma ray bursts, is thus one of the most pressing scientific endeavours in modern astrophysics. In this quest, observations offer a critical insight that both guide theoretical developments and challenge the model predications. This chapter provides an overview of the observational constraints of the multiplicity properties of OB stars obtained since 2010.