The fundamentals of high-performance liquid chromatography (HPLC), as applied in small-scale studies of drug stability, are presented. Chromatography is the separation of a complex mixture into its individual compounds through partitioning between a mobile phase and a stationary phase. A high-performance liquid chromatograph consists of mobile-phase reservoirs, pumps, a mixer to mix the solvents, a valve into which the sample is injected, a guard column, a column containing the stationary phase, a detector, and a recorder. Once compounds have been separated in the column, they pass into the detector, where an electronic signal corresponding to the amount of compound present is recorded as a peak in a chromatogram. The most common detection method is ultraviolet and visible light spectroscopy. Key concepts in HPLC theory are retention time, the time from injection of the sample to detection of a peak; capacity factor, a measure of retention corrected for the elution of an unretained compound; resolution, a measure of how well two peaks are separated; the selectivity of the method; efficiency, or resolving power; and the degree of symmetry of the peaks produced. Most HPLC separations are performed in the reverse-phase mode, which involves a nonpolar stationary phase and a largely polar mobile phase. Other modes are normal phase, ion exchange, and size exclusion. Before a drug stability study is carried out, an HPLC method must be developed that suits the needs of the proposed experiment. A thorough literature search is essential. Literature procedures serve as useful starting points but may require a great deal of manipulation. After the HPLC separation has been performed, it is necessary to validate the method used. It must be proved that the method is stability indicating, that the chromatographic standards were properly prepared, that the standard curve is acceptable, and that the method is both precise and accurate. Pharmacists who ensure that reliable, reproducible HPLC methods are used throughout studies of drug stability will obtain sound data that may be of great value in pharmacy practice.