Apache Harmony is an incubator project of the Apache Software Foundation building a compatible implementation of Java™ SE 5. The project's goals include creating an open, modular architecture that supports interoperability between multiple implementations of the Java Virtual Machine (VM), and multiple implementations of the Java SE class library. Defining a strong modular architecture enables a disparate set of Eclipse-based developers to collaborate on producing a compliant Java implementation. In this talk I will describe techniques for defining the major functional areas of Java SE and give a status report on the implementation’s modularization effort to date. I'll share some of the technical lessons learnt in the Apache Harmony project to facilitate interoperability between open source and proprietary Java VMs and class library code. I'll also demonstrate how the VM interface and class library architecture achieves the right degree of independence whilst ensuring implementation choices are not compromised. This talk is of interest to practitioners and developers with an interest in open source innovation around the Java platform, and includes a demo of the Harmony code to date.