This master's thesis focuses on how conference interpreters at the European Parliament plenary sittings deal with the use of structurally and semantically complex lexical units. Idiomatic language is influenced by culture and emotion and is usually hard to translate, which is especially reflected in interpreting, it being an extremely difficult cognitive activity on its own. Based on videos of the EP plenary sittings, a corpus was formed, made up from original speeches in English and interpreted speeches in Slovenian from which structurally and semantically complex lexical units were extracted, as well as their translations. Lexical units were classed into four categories: idiom, collocation, semantically complex free word combination and semantically complex lexical unit. Their translations were analyzed according to strategies used by Slovenian interpreters: equivalent translation, paraphrasing, wrong translation and omission. Our findings show that native speakers use more structurally and semantically complex lexical units than non-native speakers, and women use more of them than men. The majority of the analysed expressions were collocations (49 %) and idioms (44 %); the most used strategy was paraphrasing (45 %). This research offers an insight into the use of expressive language in an institutional environment and the level of interpreting of such language. Interpreters did not struggle with these expressions and even though equivalent translations often do not exist, interpreters used paraphrasing, especially in translating idioms. In interpreting, the strategy of paraphrasing is generally used almost instinctively, which is very useful in intepreting structurally and semantically complex lexical units. Students of interpreting should learn to recognize complex lexical units with idiomatic meaning during their studies to avoid missing them in a speech, and to reformulate such lexical units if a similar expression does not exist in the target language.