Educational and psychological measurement has been a main area of work for the Australian Council for Educational (ACER) since its inception. The theoretical and practical contributions of latent trait measurement and commentary on the relatively recent use of these models in Australia were the focus of a seminar celebrating the 50th anniversary of ACER. The papers include; "Educational Measurement - Theory and Practice" by Robert L. Thorndike; "Comparing Latent Trait with Classical Measurement Models in the Practice of Educational and Psychological Measurement" by John A. Keats; "The Use of Latent Trait Models in the Measurement of Cognitive Abilities and Skills" by Bruce Choppin; "The Linear Logistic Test Model and its Application in Educational Research" by Hans Spada and Regine May; "Using Latent Trait Measurement Models to Analyze Attitudinal Data: A Synthesis of Viewpoints" by David Andrich; "Conditional Inference in a Generic Rasch Model" by Graham A. Douglas; "The Use of Latent Trait Models in the Development and Analysis of Classroom Tests" by John F. Izard and John D. White; "The Use of the Rasch Latent Trait Measurement Model in the Equating of Scholastic Aptitude Tests" by George Morgan; "Some Alternative Approaches to the Improvement of Measurement in Education and Psychology: Fitting Latent Trait Models" by Roderick P. McDonald; and "A Perspective on the Seminar" by Donald Spearritt, the chairman's summary of discussion. Reactant statements were contributed by Barry McGaw; Kevin F. Collis; Glen A. Smith; Glenn Rowley; Charles Poole; and Alan G. Smith. (CM)