This chapter discusses teaching based upon pupils’ previous knowledge. As the world becomes more and more globalised, it is harder and harder for a teacher to form a picture of the pupils’ knowledge when planning instruction. However, without this information about pupils, it is impossible to know if the created learning situations are conducive to learning or if the pupils had already acquired knowledge about the learning object before the learning situation. In this study pupils’ previous knowledge is investigated in relation to how English as a Second Language is learnt when pupils have different mother tongues. In a phenomenographic study we found that pupils with three different mother tongues, when placed in the same learning situation, made errors which could be traced back to the structure of their mother tongue. This observation led to a learning study, in which variation theory was the theoretical point of departure, and in which three different research lessons with three different groups of pupils were carried out. The learning object was dependent possessives, and the pupils’ mother tongue was Swedish. As there is no differentiation between independent and dependent possessives in Swedish, the pupils could not discern the difference between these two forms. As Swedish has a differentiation between t- and n-gender, the puils wrongly assumed that the difference between my and mine was not connected to dependent and independent possessives but to gender. The results of our study show how teachers who are familiar with the pupils’ previous knowledge (mother tongue) can become aware of what mistakes they might make as ESL learners; this knowledge has a profound effect on instruction. In this way, teachers can also predict and plan what information is needed to develop learning situations which provide maximum opportunity to learn. They also understand what kinds of critical aspects are necessary to enable pupils to discern. As a result, teachers are more effective, which is reflected in better pupil results in the classroom.