Jess, the Java Expert System Shell, provides a rich and flexible environment for creating rule-based systems. Since it is written in Java it provides platform portability, extensibility and easy integration with other Java code or applications. The rules of Jess allow one to build systems that reason about knowledge that is expressed as facts. However, these facts and rules cannot capture any uncertainty or imprecision that may be present in the domain that is being modelled. This paper describes an extension to Jess that allows some forms of uncertainty to be captured and represented using fuzzy sets and fuzzy reasoning. We describe the NRC FuzzyJ Toolkit, a Java API that allows one to express fuzzy concepts using fuzzy variables, fuzzy values and fuzzy rules. Next, we describe a Java API called FuzzyJess that integrates the FuzzyJ Toolkit and Jess. Finally, we show the modifications that were made to the Jess code to allow this extension (and others with similar requirements) to be added with modest effort and with minimal or no impact as new releases of Jess are delivered.