Content and language integrated learning (CLIL), an educational approach in which subject matter and a foreign language – predominantly English – are taught and learnt side by side, has developed into a very popular educational innovation in most European countries. A host of research studies have shown its benefits, and discuss favourable effects especially with respect to L2 gains. However, critical voices have underscored the fact that CLIL attracts or selects mainly high-achieving learners. Hence, the question arises whether it is justified to attribute improved L2 performance mainly to the CLIL intervention, or to favourable learner characteristics. Several reviews of literature were published in the past, but due to a lack of longitudinal findings no conclusive evidence about the added value of CLIL in the process of L2 learning could be produced. The present review aims to fill this void and has undertaken a search of two decades of longitudinal studies into the effects of CLIL on various linguistic skills in the field of English as a foreign language. The findings indicate that robust studies were undertaken in only a limited number of European countries, and that only a few of them were large scale. Yet, the conclusions provide clear indications regarding the contexts in which CLIL leads to significantly better L2 results.