A two-part test and interviews were carried out in order to find out i) how familiar a group of 15-year-old students were with a selected set of English idioms, ii) what obstacles there were when they tried to interpret unknown idioms, and iii) if there was a difference in understanding when an idiom occurred on its own and when it occurred in a sentential context. The results show that students used different strategies in trying to interpret the meaning, such as mixing words, mixing idioms – both English and Swedish ones –, relying on encyclopaedic knowledge and giving literal interpretation. A linguistic context played an important role in interpreting the meaning of an idiom.