Instrumentation is dynamically added to application components during runtime. An interestingness engine receives inputs from various sources which identify software components such as classes for which it is desired to provide instrumentation. The sources can include a heuristics engine which receives performance data from already-instrumented components, a user interface from which a user identifies component to be instrumented or de-instrumented, and a static list of components to be instrumented. During the runtime, instrumentation can be added to components which become interesting, or removed from components which become not interesting. When loading or redefining a component, if a component type matches rules, it is eligible for instrumentation, and the interestingness engine is consulted to determine whether the component is of interest, in which case instrumentation is added. If a component type does not match rules, but the component is interesting, the rules can be updated to include the component type.