The Ptolemy project studies modeling, simulation, and design of concurrent, real-time, embedded systems. The focus is on assembly of concurrent components. The key underlying principle in the project is the use of well-defined models of computation that govern the interaction between components. A major problem area being addressed is the use of heterogeneous mixtures of models of computation. A software system called Ptolemy II is being constructed in Java. Ptolemy II differs from other commonly used graphical block-diagram languages in that they typically support only one model of computation. In addition, Ptolemy II is a more open architecture in that its infrastructure is open source, and the interfaces to the core mechanisms in the software are published and documented. The Ptolemy project has been under development in Java since 1997. The work is conducted in the Department of Electrical Engineering and Computer Sciences of the University of California, Berkeley. Currently, full-time members of the project include 1 director, 3 system/sw managers, 5 postdoctoral/visiting scholars, 11 grad students, 2 visitors, 2 undergrad students (details at http://ptolemy.eecs.berkeley.edu/people/main.htm ). Our contributors include people from industry, universities, research institutions and other public projects (details at http://ptolemy.eecs.berkeley.edu/archive/links.htm ). We have a long collaboration with the Kepler project ( http://www.kepler-project.org ), an open-source scientific workflow system that uses Ptolemy as its core.