The grammar of focus displays a wide range of multimodular effects (in semantics, syntax, morphology, and phonology). The classical analysis of focus stemming from the Principles & Parameters framework considered it as an essentially syntactic phenomenon (with interface consequences), and this is still the core idea under the cartographic approaches such as Rizzi’s (1997). However, because of this multimodular nature, alternative approaches have been proposed that conceive of focus as an interface phenomenon that emerges from the interaction of different modules (e.g. Zubizarreta’s 1998 and Reinhart’s 2006 Nuclear Stress Rule-based approaches). Recently, other types of modular interactive proposals have been made that dissociate focus from nuclear stress placement (e.g. Struckmeier 2017 and Titov 2020). In this chapter I criticize such approaches and make a plea for the syntactic nature of focus as the only possible conception with explanatory power. In doing so, I provide evidence from a wide range of phenomena and languages suggesting that focus marking has to be a syntactic phenomenon, with a variety of effects and externalization patterns attested cross-linguistically.