If you plan to enter or leave the EU with € 10 000 or more in cash you must declare it to customs. Customs authorities are empowered under the Cash Control Regulation to check persons, their luggage, and their means of transport - and detain cash that has not been declared. The € 10 000 threshold is high enough so as not to impose obligations on the majority of travellers and traders.
Be careful ! Cash is not only currency. Other items are also considered cash.