Over the last 15 years, high-performance liquid chromatogra- phy LC has made the transition from an instrument used only by experts in research labs to a tool used for routine applications by relatively unskilled workers. With this transition have come in instrumentation and column technology. In major advances the past, the operator had to be a jack-of-all-trades, with a screw- driver, soldering iron, and various wrenches as constant compan- ions in the LC lab. Today, many instruments contain micropro- cessors as powerful as those of mainframe computers of earlier days. With this technology has come a variety of self-diagnostic tools that allow the LC system to locate many of its own prob- lems. Traditionally, well-honed LC troubleshooting skills have been a result of years of work at the bench. Today the LC system itself often can do a better job of troubleshooting than the operator can. Yet many of the problems of the past are still the major problems of today air bubbles, check val