"Recent studies on the syntax and semantics of complex sentences have dealt with several challenges to the traditional boundaries between coordination and subordination. Some constructions belong to one of the two types according to syntactic criteria but relate to the other type on semantic grounds, whereas other constructions are not compatible with either the canonical syntactic or semantic tests traditionally employed to establish this distinction. Other constructions, by contrast, seem to have evolved in such a way that they now cross the divide between both types. The collection of papers in this volume delves further into the theoretical implications of previous analyses and focuses on a wide array of data from different languages, taking those challenges as a point of departure to develop innovative perspectives and to advance thought-provoking ideas.