Most Java programmers use some kind of tracing system to keep track of potential errors and problems in code that is under development. However, multithreaded and multiplatform systems can generate a large and baffling amount of tracing data. This article offers some tips that will help you make sense of trace data from complex applications. You'll learn how to use log4j, an open source logging package, to generate information-rich log files. You'll also see how you can use standard UNIX shell commands to mine that data for the information you need.