One of the several causal mechanisms for errors made by adult second language learners is interference from the native language. This paper attempts to account for the cognitive nature of interference by proposing two psycholinguistically based principles that will explain various types of production and perception errors made of second language learners. The operation of the principles is then documented with examples from three different language learning contexts: native speakers of English learning Hausa yes - no questions, Persians learning English nominal compounds and relative clauses, and native speakers of English learning to read German. (Author)