This document discusses auditing reactive applications to detect blocking API calls. It describes how blocking calls can negatively impact performance in reactive systems by consuming thread pools. Various techniques for detecting blocking calls are examined, including modifying the JDK, generating warnings during compilation, and instrumenting code at runtime using a JVM agent. Aspect programming is highlighted as a way to audit applications at load time by weaving in checks for over 500 blocking methods across many Java APIs. The reactive-audit tool is introduced as an open source project for helping developers test for blocking calls in frameworks like Play, Jetty, and Akka.