Given some test case, a program fails. Which circumstances of the test case are responsible for the particular failure? The Delta Debugging algorithm generalizes and simplifies some failing test case to a minimal test case that still produces the failure; it also isolates the difference between a passing and a failing test case. In a case study, the Mozilla web browser crashed after 95 user actions. Our prototype implementation automatically simplified the input to 3 relevant user actions. Likewise, it simplified 896 lines of HTML to the single line that caused the failure. The case study required 139 automated test runs, or 35 minutes on a 500 MHz PC.