ility concepts in abstract data type specification, 30 years later Donald Sannella1 and Andrzej Tarlecki2,3 1 Laboratory for Foundations of Computer Science, University of Edinburgh 2 Institute of Informatics, Warsaw University 3 Institute of Computer Science, Polish Academy of Sciences We recall the contribution of Montanari's paper [GGM76] and sketch a framework for observable behaviour specification that blends some of these early ideas, seen from a more modern perspective, with our own approach.