In September 2006, preliminary to the IWA biofilm conference, a second workshop about aerobic granular sludge was held in Delft, The Netherlands, of which a summary of the discussion outcomes is given in this paper. The definition of aerobic granular sludge was discussed and complemented with a few additional demands. Further topics were formation and morphology of aerobic granular sludge, modelling and use of the aerobic granular sludge in practice.