The purpose of this chapter is to better understand the challenges preliterate adults have to deal with in simultaneously learning a new, additional language and learning to read and write in that very language. The chapter gives an overview of the most important research findings on the effect of literacy on cognition and L2 learning, on adult L2 literacy acquisition and L2 literacy teaching of LESLLA learners. Research reveals that literacy triggers the development of several metalinguistic and information processing skills that often are taken for granted in mainstream SLA theories. Research on beginning reading and writing clearly indicates that literacy is not acquired spontaneously. It requires explicit and stepwise attention for the relationship between written signs and the phonological constituents of the language, because unlike a beginning reader in L1 a LESLLA learner cannot build upon already existing semantic and sound representations of words, nor, like a literate L2 learner, upon L1 literacy skills and multiple strategies. Research on teaching reveals that L2 literacy teaching and empowering LESLLA learners is an enduring task that requires careful attention for code-learning, vocabulary and adult-oriented, motivating and relevant content.