To introduce this Special Issue entitled Clausal and Nominal Complements in Monolingual and Bilingual Grammars, we begin by explaining what originally motivated this Special Issue. The first two co-editors (Ji Young Shim and Tabea Ihsane) worked on the research project entitled Selection at the Interfaces, in which various linguistic aspects (e.g., syntactic structure, interface with semantics, etc.) of clausal and nominal complements in monolingual grammars were explored. 1 To extend an investigation of these issues to bilingual contexts, they organized a two-day workshop entitled Clausal and Nominal Complements in Monolingual and Bilingual Grammars in June 2016, where the third co-editor (M. Carmen Parafita Couto) of this Special Issue was an invited speaker. 2 The workshop aimed to investigate the left periphery of complements, in particular the left periphery of the clause and the nominal phrase and its edge, such as C(omplementizers) and D(eterminer) and other topmost functional layers, where languages may be parametrized differently, thus leading to linguistic variation. Within generative grammar, it has long been assumed that language variation is due to variation in the domain of functional categories and their morpho-syntactic properties [1,2]. Following this tradition, the left periphery of the clause has been extensively investigated, confirming the hypothesis that the functional category C(omplementizer) and its morpho-syntactic properties may be parameterized differently across languages within research on monolingual grammar [3-6]. In addition, the left periphery of a nominal phrase has also been investigated to a great extent [7-9], based on the proposal that clauses and nominal phrases have parallel structures ([10] and in subsequent work). In recent years, generative linguists have also started to pay attention to the left periphery of bilingual grammars, especially in relation to diverse patterns of code-switching, which is the mixed use of two or more languages in conversation, and which is frequently observed in bilingual speech. Under the assumption that monolingual and bilingual grammars are subject to the same grammatical principles [11-14], several researchers have investigated the grammar of code-switching in various language pairs and showed that the left periphery of a particular functional category such as C, D, or v may be parameterized differently across languages and derive certain patterns of code-switching, 1 The projected was funded by the Swiss National Science Foundation (#100012_146699/1), and the second Editor was the principal investigator of the project. 2 The workshop was funded by the University of Geneva and the Swiss National Science Foundation.