Book historians have generally seen the introduction of the printed book auction catalogue as an important event in the history of the book trade. Catalogues were already being printed in the Dutch Republic in about 1600 and the present article discusses the factors that favoured this remarkably early development. In section 2 the author surveys present knowledge of book auctions from classical antiquity up to the year 1598. In particular, he discusses sales of books in the estates of deceased persons in the Low Countries during the fifteenth and sixteenth centuries, with particular reference to auctions in Leiden and The Hague in the last part of the sixteenth century. From the data assembled it emerges that the auctioning of books was certainly not first thought of in the Dutch Republic and that many auctions of property, including books, were held before 1599. In 1596 Louis (II) Elzevier was granted permission to hold book auctions in the Great Hall of the Binnenhof in The Hague, and in the hands of a bookseller it was possible for this form of trade to develop in the best possible way. In section 3 the author moves on to the earliest book sales with printed catalogues, namely the Marnix sale of 1599 and the Daniel van der Meulen sale of 4 June 1601 and following days. In the latter case it is possible to establish that the initiative to print a catalogue came not from the bookseller but from the deceased's widow and executor. Their instructions to the Leiden bookseller Louis (I) Elzevier were merely to conduct the auction, for which service he received five per cent of the proceeds. These large book sales in Leiden were apparently so successful that the inheritors of scholarly libraries in other towns had them auctioned in Leiden. After Leiden, sales with printed catalogues were also held in two other towns: in The Hague in 1605 and in Middelburg in 1605-7. From the catalogue of the auction held in Middelburg on 26 February 1607 it is clear that the collection was not a private one but either a part or the whole of a bookseller's stock. In 1606 and 1607 auctions were held in Leiden to sell some large stocks of books, namely those of the deceased Heidelberg printer and publisher Hieronymus Commelin. Some of these books, however, came from still living members and partners of the Commelin family, and when in 1608 the same group of wealthy dealers again wanted to have large numbers of unbound books auctioned in the town the booksellers of Leiden asked the authorities to ban auctions of this kind on the grounds that they were doing serious damage to the local book trade. Between 1607 and 1610 at least sixteen auctions with printed catalogues were held in Leiden, of libraries both of Leiden scholars and clerics and of owners from elsewhere. Only one catalogue from The Hague is known from this period (1609). One noteworthy element is the 'Appendix' to be found in some of the catalogues. The books in this section were, as the author shows, probably not privately owned but from the auctioneer-bookseller's own stock. He was taking the opportunity to turn part of his stock of books into hard cash. In section 4 the author describes the circumstances in Middelburg, The Hague and Leiden that help explain why book auctions first developed in those three places. After Amsterdam, Middelburg was at this time the most important trading centre in the Dutch Republic, and many merchants from the Southern Netherlands had settled there in the first decade of the seventeenth century. The book collections auctioned in Middelburg were modest in size. However, the number of potential buyers was probably also quite small, certainly compared with Leiden. Circumstances in The Hague were sharply different from those in Leiden and Middelburg. The book auctions were held on land belonging to the Court of Holland, where the regulations drawn up by the town and the rules of the Hague guild did not apply. In the Great Hall of the Binnenhof Louis (II) Elzevier was allowed to hold as many auctions as he liked. The climate was favourable to him, what with the presence of the many delegates to the States of Holland and the States General, the officials working for the many administrative and legal bodies and his fellow booksellers in the Hall. This was particularly true for law books, but in the period described by the author Leiden became the undisputed centre of auction sales of scholarly libraries. The author regards the following factors as of decisive importance. (1) In contrast to other towns in the province of Holland, such as the important book centre of Amsterdam, the Leiden booksellers could themselves act as auctioneers and collect five per cent of the proceeds as their fee. (2) In about 1600 Leiden was the only town in the Republic where the booksellers were not organized into a guild. In practice this meant that there were no rules for the booksellers to have to observe when holding auctions. Furthermore, the auctioning of large and important collections was in the interest of the university community, and the university governors consequently did their best to block any attempt to introduce local regulations which would prejudice the auctions. (3) The university was attracting ever larger numbers of students, both from the provinces of the Dutch Republic and from the surrounding countries. They not only bought for themselves, but also on behalf of friends and relatives in other places. Besides these crucial factors the author also discusses some other favourable circumstances. For example, the whole development of the book auctions took place against the background of extremely rapid economic growth and a great many other new initiatives in the commercial field. In the early decades of the seventeenth century the book trade in Leiden enjoyed almost unlimited freedom of action. The innovations in auctioning books gave the town an important lead over the country's other centres of the book trade. Finally the author turns to two important consequences of the introduction of the printed catalogue. Using a catalogue it became possible to reach so many potential customers that booksellers were now able to turn their own stocks into hard cash for acceptable prices through the medium of the auction. It was thus possible to some extent to nullify one of the disadvantages of the existing system, the 'Change', which tended to produce large, not to say too large, stocks. And in the second place the introduction of the printed catalogue had a particularly stimulating effect on the formation and enrichment of both institutional and private libraries in the seventeenth century.