This document discusses different types of language input that can contribute to second language acquisition. It identifies three main types: pre-modified input, which is language input that has been simplified before being provided to learners; interactionally modified input, which emerges through negotiation of meaning between interlocutors to aid comprehension; and modified output, which occurs when learners modify their language production in response to interaction. The document reviews research supporting these three types of comprehensible input and their role in facilitating second language acquisition. It also notes that while input is widely seen as important for SLA, theories differ in how central a role they attribute to input.