Many previous CALL systems lacked reliable feedback pointing to specific production problems. Even if pronunciation errors are pointed out, it is almost impossible for learners to correct their pronunciation errors without a supervisor's help. In order to provide such instructions by machine, several points should be first considered. First, it is important to make the learners aware of in what way their production deviates and show them how to correct it. It has been said, ``no awareness no progress''. Commonly, this can be done through contrastive instruction. That is, the split screen shows simultaneously the correct model of the native speaker and the deviant production of the learner. Instruction is based on this comparison. Secondly, many investigations suggest that auditory feedback has much effect upon speech production and that auditory feedback is closely linked to speech production. Generally speaking, when the auditory feedback is incorporated into feedback instruction, it can accelerate the correction of production error. The learners can find their pronunciation problem by means of replaying the contrast of the model speech and their speech. Besides, the following methodologies should be considered for an effective L2 pronunciation instruction.
