This rapid spread of Content and Language Integrated Learning (CLIL) across Europe means that secondary education teachers of subjects like science or history are increasingly required to teach their subjects in English. For some this is a positive impulse to professional development, but in all cases changing the language of instruction poses a challenge, sometimes triggering negative feelings and leading teachers to question their professional identity. After reviewing relevant research, we analyse the impact of a training programme for in-service CLIL teachers on their emotions and identity. Our findings show that the transition to CLIL was a cause of considerable insecurity in this sample, associated with threats to the teachers¿ professional self-confidence and satisfaction. Future training should allow more space for consideration of emotional factors, with a view to helping teachers reshape their professional identity.