A hypothesis that learners will acquire language best when the material is comprehensible to them. The input should be accessible so that they can understand it, but it should be just beyond their level of competence. Input will lead to acquisition so long as the input is challenging, yet easy enough to understand without conscious effort at learning. If the learner is at level i, then input should come at level i+1. I+1 means that material is comprehensible with a few new forms for students to naturally acquire.