The concept of a learner's grammar is introduced. Then the linguistic and psychology of learning considerations underlying the concept are presented and discussed. The linguistic considerations concentrate on the question of how a learner's grammar basically differs from other (linguistic grammars and what consequences arise from this. The learning psychology considerations aim primarily at discovering the conditions under which the recognition, comprehension, retention and application of language (grammatical) regularities are facilitated or impeded or prevented; here the findings of memory and brain research play an important role.