In binary systems consisting of two massive stars, the interaction of
the powerful stellar winds of the two components produces signatures
over a broad range of wavelengths. Many observational and theoretical
studies of this phenomenon have been performed over the last two
decades. In very close massive binary systems, mass exchange due to
Roche lobe overflow can also occur and the signatures of
... [Show full abstract] the two
phenomena are sometimes hard to distinguish. In this review, I discuss
some recent developments in the study of both phenomena.