Language processing research is changing in two ways that should make it more relevant to the study of grammatical learning. First, grammatical phenomena are re-entering the psycholinguistic fray, and we have learned a lot in recent years about the real-time deployment of grammatical knowledge. Second, psycholinguistics is reaching more diverse populations, leading to much research on language processing in child and adult learners. We discuss three ways that language processing can be used to understand language acquisition. Level 1 approaches (“Processing in learners”) explore well-known phenomena from the adult psycholinguistic literature and document how they play out in learner populations (child learners, adult learners, bilinguals). Level 2 approaches (“Learning effects as processing effects”) use insights from adult psycholinguistics to understand the language proficiency of learners. We argue that a rich body of findings that have been attributed to the grammatical development of anaphora should instead be attributed to limitations in the learner’s language processing system. Level 3 approaches (“Explaining learning via processing”) use language processing to understand what it takes to successfully master the grammar of a language, and why different learner groups are more or less successful. We examine whether language processing may explain why some grammatical phenomena are mastered late in children but not in adult learners. We discuss the idea that children’s language learning prowess is directly caused by their processing limitations (‘less is more’: Newport, 1990). We conclude that the idea is unlikely to be correct in its original form, but that a variant of the idea has some promise (‘less is eventually more’). We lay out key research questions that need to be addressed in order to resolve the issues addressed in the paper.