Most of our understanding of stars and how they evolve is based on the assumption that they are completely isolated in space, never interacting with one another. However, studies over the last decade have shown that many more stars than we thought exist in gravitationally-bound binaries, triples, and even larger groupings. The addition of companions increases the complexity in the systems, but also opens up the possibility for interactions between the stars and the formation of unusual astronomical objects. I will discuss the current state of the field, and some of the challenging open questions that continue to puzzle scientists. Presented 22 April 2022.