A client-side business rule engine for executing business rules, at a client, in a rich internet application is disclosed. A set of business rules and a fact base are compiled at a server in accordance with a platform definition model, such that the compiled rules and facts are formatted to conform for use with a business rule engine based on the platform specified in the platform definition model. The business rule engine, embodied in a code module, is served to a requesting web browser application along with a subset of the compiled business rules and facts. At the client, the business rule engine evaluates conditions expressed in the business rules in light of facts received from the server, and facts derived at the client.