This volume explores first language influences on multilingual lexicons. We begin, when we consider what is involved in the learning process, by outlining that outdated pursuits of language learner (bilingual) goals to mirror “native” like (monolingual) users is unrepresentative. Ortega (1996) cites Slabakova’s (2013, pp. 53–54) concern that this comparison is “like comparing apples and pears” because “the input of bilinguals and monolinguals is too varied for direct comparisons to be justified” (p. 53). We agree with Ortega and Slabakova to the extent that such comparison fails to accurately characterise the learning process, and so while our aim is not to provide a definitive picture of this multifaceted relationship, we do hope to show that the relationship is far from straightforward than previously thought. In exploring similarities and differences between the L1 and additionally learned languages, we suggest that researchers can then identify what might be more easily learned (the similarities) versus the more difficult (the differences). In discussing these similarities and differences with vocabulary as a central thread throughout the book, we do acknowledge that interaction between first and additional is likely influenced by additional factors (e.g., “proficiency”; Kroll & Stewart, 1994; Kroll et al., 2010). Accordingly, we focus on vocabulary but also consider additional factors in presenting a series of empirical studies that begin to show the L1 influences on multilingual lexicons.