© 2018 selection and editorial matter, Elabbas Benmamoun and Reem Bassiouney; individual chapters, the contributors. The poet Hafez Ibrahim has a memorable line in his famous poem on the Arabic language. In that line, Arabic boasts that it is a sea whose depths contain treasures and then wonders whether the diver has been asked about them. For modern linguists, that line applies to all natural languages. Though there has been extensive research on many languages from many regions of the globe, there are still too many unanswered questions and still many depths to plumb. What makes research on natural language challenging is its inherently multifaceted character. Language is a human faculty that can be acquired by both children and adults, and can get impaired. Those attributes engage psychology and neuroscience. Language also reflects social stratification and the dynamics of social interactions and relations, properties that engage fields such as Sociology and Anthropology. Unlike other cognitive faculties, individual languages undergo change, some of which is due to contact with other languages. The latter properties depend for their analysis on knowledge of history, population movement, and intimate familiarity with the languages in the contact situation. Language can also be modeled computationally, and due to advances in information technology we now have tools that can, with varying degrees of success, recognize and produce language. However, the most obvious property of language is that it is a means for communication and artistic expression. The communicative function of language is carried out through sounds, signs, words, and longer expressions, such as phrases, sentences, and extended discourse. These overt manifestations of language can also vary between languages but may display properties that are similar, raising questions about their nature and what they reflect about human cognition. Unfortunately, research on languages has been uneven, mostly due to lack of resources and expertise. Some languages, particularly English, have received extensive attention and have been explored from the different angles mentioned earlier. Other languages, however, have not been as fortunate - and some, including some Arabic varieties such as Sason Arabic discussed by Akkus in Chapter 25 - may never get that chance because they may become extinct in a few generations. The majority of Arabic varieties, including Standard Arabic, falls somewhere in between. Some aspects of the Arabic language have long featured prominently in linguistic research going back several centuries to the Arabic linguistic tradition. That research focused particularly on the sounds patterns of Arabic, word formation, some aspects of syntax and semantics, and dialectal/regional variation. Other aspects of Arabic have started getting the attention of the linguistic community only in the last century and early in this century. This handbook 2aims to take stock of where the research stands in many of those areas. The chapters in this volume aim to provide the reader with an overview of the state of the research in various areas of Arabic linguistics, describe the results and the research that led to them, and point to future directions. We could not do justice to all the areas of Arabic linguistics but we have tried to focus on research that has enriched the debates on Arabic and its varieties while also contributing to larger questions about natural language in its different manifestations, either because Arabic displays some properties that shed further light on some complex general issues, such as subject verb agreement, negation, tense, syllabification, acquisition of heritage Arabic, etc., or where Arabic can highlight properties that are not as well-known crosslinguistically, such as diglossia, the role of the consonantal root in word formation, and experimental and computational approaches to a language with a root and pattern system.