Following Bowles's (2010) meta-analysis of reactivity research regarding think-alouds, and Jiang's (2012) overview of reaction time methodologies, the book under review is a new addition to the Routledge Series on Second Language Research Methods, devoting to an online data collection method, namely eye-movement registration, commonly referred to as eye tracking.Eye tracking is primarily used to detect and measure an eye's movements (saccades) and stops (fixations), as well as movements back in a text when reading (regressions) (Conklin & Pellicer-Sánchez, 2016). This new technology is becoming increasingly popular in applied linguistics, second language acquisition (SLA) and bilingualism, to investigate questions of language processing and representation using on-line measures, in that the data obtained in this way offer a more fine-grained representation of the learning process than any off-line measurements could (Godfroid & Schmidtke, 2013). The turn toward eye-tracking methodology is a part of a larger movement in SLA research that emphasizes the use of concurrent data collection methods. The growing interest in eye tracking is evidenced by an increasing number of studies making use of this methodology in recent years. Since the first special issue on eyemovement recordings in 2013 in Studies in Second Language Acquisition, researchers in more and more subdisciplines in second language (L2) research have taken up eye tracking, and it has become a popular tool within SLA and bilingualism. The latest special issue on eye tracking in 2020 in Second Language Research is a reflection of these developments, as it shows the breadth and wealth of contemporary eye-tracking research in L2 processing in dialogue with its neighboring disciplines.However, before we get too far down the road with eye tracking, it is important for the field to come to understand such questions as what eye tracking is, and how and why to best collect and analyze eye movement recordings for welldesigned and informative language research. Luckily, answers to these questions can be found in the present book, Eye tracking in second language acquisition and bilingualism: A research synthesis and methodological guide, which aims at providing foundational knowledge and hands-on advice for designing, conducting, and analyzing eye-tracking research in applied linguistics.Over the course of nine chapters, the author presents theories, methodology and practice principles that will inform the use of eye tracking. The opening chapter introduces eye-tracking methodology in relation to three other concurrent methodologies-think-aloud protocols, self-paced reading (SPR) and event-related potentials (ERPs)-which present themselves as complements and sometimes competitors to the eye-tracking method. Such contrasts aid the understanding of what eye tracking is and is not, and how it can enrich one's research program. By summarizing the cognitive psychology literature, Chapter 2 provides the reader with a set of fundamental facts about eye movements that proba...