Designers of computer aided learning systems have long viewed their task in isola- tion from the cognitive aspect which it involves. However, in recent years, it has become clear that the success of such systems depends largely on the system having a sophisticated model of its users. The reason why such a model is mandatory is that in order to adapt the instruction to the learner's speci?c needs, it is necessary to retain a model of this learner. This model must capture different aspects of the learner's knowledge and the way it evolves and varies over time. One of the major problems in this enterprise is, precisely, the characterisation and explanation of the learning process. In the research reported below, this problem is addressed. A system designed and implemented to model learners in the course of acquisition of a second language is presented. The requirements for this system are that it be language independent and cognitively valid. The ?rst requirement has led to the selection of a unifying theory of language which encompasses all human languages in a Universal framework. Cognitive validity, on the other hand, has entailed the de?nition of a model of second language acquisition. This model provides an in- sight on how language learning takes place, by de?ning various acquisition stages and identifying the type of language, produced by the learner, which characterises each stage. The computational system ?nds support in this model to explain the learner's behaviour. The learner°s model, which consists of her knowledge of the second language and her misconceptions about it, is inferred after analysis and diagnosis of sentences that she has produced. The core of this system is at a level of generality which enables it to be applied to a particular language without the need for extra machinery. The specialisation to a language is achieved by the de?nition of some speci?c features of this language together with its lexical properties.