SLA scholars who investigate learner language seek to explain L2 competence and L2 development. Competence is deﬁned as the nature of themental representations comprising the internal grammar of learners and development refers to the processes and mechanisms by which those representations and the ability to use them change over time. The focus is typically on grammar, and more narrowly on morphology and syntax. In order to understand what it means to become a competent user of an additional language, many other dimensions of the target language system must be learned, including vocabulary, phonology, pragmatics and discourse. However, thus far SLA efforts have been most persistent and most fruitful in the L2 areas of morphology and syntax. This will be our focus in this chapter, too. In the study of learner language there are two traditions: interlanguage studies and formal linguistic studies of L2 acquisition. Even though ultimately both share the same goal of understanding learner language, each has its own constructs, tenets and preferred methodologies. In this chapter, I will familiarize you with insights about learner language gleaned by the study of interlanguage from the general cognitive learning position and I will emphasize development over competence. Throughout the chapter, I will illustrate ﬁndings and arguments with oral and written discourse data. Whenever possible, I have drawn on a variety of attested examples from a number of target languages.