Input is the name of a topic – the way that language 'out there' impacts the development of interlanguage, within the individual. It is perhaps the most important aspect of second language learning. This Element offers an overview of the key concepts related to input and the major lines of research exploring its nature and its role in second language learning. It then puts things together into a coherent, if controversial, picture of input and its role in development, emphasizing the place of consciousness. In this and most other current perspectives, implicit (unconscious) input-based learning is the heart of second language acquisition. This suggests two general options for teaching: (a) trust the natural implicit processes, trying to create optimal conditions for them; (b) direct those processes to selected features of the input, probably using explicit instruction. The conclusion is that (a) appears preferable.