When Frits Lugt purchased two of the surviving seven letters written by Rembrandt, he was laying the foundations for what would become one of the most extensive parts of the Fondation Custodia’s collection. It was primarily his successor, Carlos van Hasselt, the director from 1970 to 1994, who boosted the expansion of the modest group put together by Lugt, which centred on unique letters by great artists of the sixteenth and seventeenth centuries, into an encompassing archive with artist’s documents from all countries and all times. An active acquisition policy has seen the number of items grow to more than fifty thousand, spanning a period from the late fifteenth century to the present day. Apart from countless individual items, the collection contains a number of comprehensive correspondences and archives, and it includes writings by or documents concerning collectors, art critics and art historians. It is still being expanded and increases by some hundreds of items each year.