MathEdit is a browser-based interactive editor for mathematical expressions being developed jointly by researchers at Kent State University (USA) and Lanzhou University (China). MathEdit is an open source software written in JavaScript and DOM. It makes use of browser support, either natively or via a plug-in, for MathML display. The dynamic nature of MathEdit makes it useful for mathematical web sites to enable users to create/edit/submit mathematical expression. MathEdit is also an important tool for WME (Web-based Mathematics Education) and MAG (Mathematics Assessment Grid), both are on-going research projects at Kent and Lanzhou. We present the design and implementation of MathEdit, its user interface, program architecture, and API. Also discussed is the flexibility and extensibility of MathEdit.