The errors that Java programmers are likely to encounter can roughly be categorized into three groups: compile-time (semantic and syntactic), logical, and runtime (exceptions). While much work has focused on the first two, there are very few tools that exist for interpreting the sometimes cryptic messages that result from runtime errors. Novice programmers in particular have difficulty dealing with uncaught exceptions in their code and the resulting stack traces, which are by no means easy to understand. We present Backstop, a tool for debugging runtime errors in Java applications. This tool provides more user-friendly error messages when an uncaught exception occurs, but also provides debugging support by allowing users to watch the execution of the program and the changes to the values of variables. We also present the results of two studies conducted on introductory-level programmers using the two different features of the tool.