Please cite: Armon-Lotem, S., & Meir, N. (2019). The nature of exposure and input in early bilingualism. In A. De Houwer & L. Ortega (Eds.), The Cambridge handbook of bilingualism (pp. 193–212). Cambridge, UK: Cambridge University Press. ______________________________________________________ 1 Chapter 11. The Nature of Exposure and Input in Early Bilingualism Sharon Armon-Lotem and Natalia Meir 1 Introduction The linguistic development of young bilingual children, whether simultaneous or sequential, and regardless of whether a single modality or two modalities are involved, is affected by both child internal (e.g., level of maturation, working memory, chronological age) and child external factors. The latter include age of onset of exposure to more than one language (whether in another modality or not) as well as the quality and quantity of the language children are exposed to and other aspects of children's linguistic environments, such as whether parents or caretakers provide native or non-native input and high or low levels of code-mixed input. Our aim is twofold: to contribute towards a better understanding of the complex factors that moderate the impact of exposure and input on bilingual children's linguistic development, and to help improve measurement practices in this research domain. Rather than offer a historical review starting from the first studies published since Pearson, Fernández, Lewedeg and Oller's (1997) seminal study, we concentrate on the present state of understanding about the topic, highlighting methodological advances and key findings. We will also pay close attention to early sequential bilingualism and review many studies where exposure or input are examined. For a review of studies examining the influence of input and exposure on mainly bilingual first language acquisition with simultaneous onset of exposure to both languages from birth, readers are referred to De Houwer (2017). Measuring young bilingual children's language exposure and input is not only of interest to better understand the bilingual development process, but also to be able to evaluate whether any finding of delayed language acquisition by a bilingual child is due to limited exposure or to language impairment. However, we do not review here the research that attempts to distinguish delays in acquisition caused by language impairment versus low exposure (for more on this, see Tuller, 2015; Hammer & Edmonds, this volume). We will show that there are advantages and disadvantages in measuring exposure and input via relative exposure (including cumulative exposure), absolute exposure, and the content in the input of specific linguistic constructions. We