The primary reason for quality control is to ensure that the conclusions of the various investigators, and the summaries developed therefrom, are based upon comparable data. In addition, the program will aid investigators in ascertaining whether their needs for precision and accuracy are being met and will contribute to overall proficiency by providing opportunities for scientists to discuss solutions to common problems. A secondary reason for quality control is to ensure that data accumulated in PLUARG studies are accurate and precise contributions to the data base for future use in the Great Lakes Basin. This Handbook is intended to give investigators associated with Task Group C studies under PLUARG the guidelines for quality control.