The presence of the L1 in classes where English is taught as a Foreign Language (EFL) has always provoked debate. As a result, the role of the L1 remains a matter of discussion in the bilingual education classroom. Moreover, the presence of both the L1 and a foreign language can be controversial in programmes where both the L1 and the foreign language are used as languages of instruction as part of the curriculum. The use of the L1 in bilingual environments has been identified as potentially logical instrument to scaffold content. In Content and Language Integrated Learning (CLIL) contexts, however, the situation may be different and there is a need to provide substantial empirical evidence of real classroom practices in order to support the idea that employment of the L1 may be an enriching force, especially with students exhibiting a low proficiency in the L2. In this study we have analysed the L1 strategies in CLIL classes regarding a) type of key strategies and techniques, and b) purpose. Results indicate that students exhibit a frequent, intuitive, non-systematic use of the L1, and that the lack of proficiency in the L2 is a decisive factor in explaining this overuse. In addition, students act with a clear purpose in the L1 but their behaviour is not systematic and well defined. Finally, the use of the L1 does not seem to affect the learning of content negatively, although it obviously reduces the time students devote to the use of the L2.