This lecture, given at the Kunsthistoriches Museum, Vienna, during the great Bruegel exhibition of 2018-2019, explores the hidden techniques used by Pieter Bruegel the Elder in the creation of his paintings, in particular the Battle between Carnival and Lent of 1559. It provides, for the first time, conclusive evidence for the use of cartoons in the preparation of his paintings. The lecture also explores the fascinating phenomenon of the copies of Bruegel the Elder's paintings, which were reproduced in vast numbers by his elder son Pieter and to a lesser extent his younger son Jan at the end of the sixteenth and first part of the seventeenth century. It reveals precisely how the sons were able to imitate their father’s works with such exactitude, even when they couldn’t see them with their own eyes. The lecture also touches on the tricky subject of attribution. In the seventeenth century, such was the renown of Bruegel the Elder that many anonymous imitators made their living by reproducing his popular scenes. Sometimes it takes a forensic approach to distinguish between Pieter Brueghel the Younger’s copies, those of his workshop assistants, those of his contemporaries and those of later imitators.