The 13th European Conference on Object-Oriented Programming (ECOOP 99) brought some 500 participants to Lisbon from June 14th to June 18th, 1999. As usual, the workshops took place during the rst two days of the conference and gave authors and participants an opportunity to present and discuss the most topicalandinnovativeideasinobject-oriented technology.Theimportanceofthe workshopswithinthecontext ofECOOPisbecomingincreasinglyrecognised; for the rs t time inthe history of the conference, the number of workshopproposals for ECOOP 99 actually exceeded the slots availableand some had to be refused. In addition to the usual conference proceedings, Springer-Verlag has also undertaken, forthe pasttwo years, the publicationof a WorkshopReader, which bringstogether the results of the workshops, panels, and posters held during the conference. This book, the 4th ECOOP WorkshopReader, di ers from previous editions in two signi cant ways. Firstly, instead of simply reproducing the position - pers, it presents an overview of the main points made by the authors as well as a summary of the discussions that took place. Secondly, to make the text more uniformandreadable, allchapters havebeen written ina commonformat(using LaTeX lncs style les). This book was only possible thanks to the eo rt of each workshop organiser in particular, and each workshop, poster, and panel participant in general. The innovations introduced in this book implied additional work for the workshop organisers in terms of recording and summarising the discussions as well as adapting the written presentations to a common format. This extra e ort will certainly be appreciated by the readers.