As biological wastewater treatment plants become ever more complex, their successful control becomes increasingly critical. Furthermore, the introduction of biological phosphorous uptake as an element in the biological processes of activated sludge treatment plants has created the need for deterministic modelling of these processes. Thus the success of an activated sludge plant depends on both modelling and control. Modelling and control are normally dealt with in three separate fields - modelling of settlers, modelling of biological processes and control of the whole system. The idea behind the specialized IAWQ Seminar held at Kollekolle, near Copenhagen, was to bring together these three fields. It also introduced the new "IAWQ Activated Sludge Model No.2: Biological Phosphorous Removal". From these proceedings it can be seen that the seminar fulfilled the objectives set. The papers published here give an up-to-date account of these fields and of how they can be integrated, and indicate in which direction research is moving. In the same way as the two previous (1986 and 1992) Kollekolle proceedings, this volume should prove invaluable as a solid reference of the current state of the art.