This chapter, after a brief introduction, provides an overview of how the two notions of synchrony and diachrony have been analyzed in the literature (sect. 2). Section 3 takes into account the main types of phenomena that may be considered as manifestations of the synchrony-diachrony interface; Section 4 focuses on theoretical and methodological issues, examining those approaches and tools that may better capture interface phenomena, finally Section 5 provides a brief summary of each chapter, underlining those respects in which every paper within tho book constitutes an original contribution to the overall debate on the relations between synchrony and diachrony.