This manual covers the principles, use, limitations, and applications of colorimetric detector (indicator) tube systems. Colorimetric tubes change color and provide quantitative measures of targeted contaminants present after sample gas is drawn through the tube, typically with a hand pump. Color change can manifest itself in the length of a color stain or a change in color intensity. Colorimetric detector tube systems are quite simple, easy to use, and relatively inexpensive, compared with instrumental methods of measuring airborne concentrations of toxic materials. Their simplicity of operation, in fact, could result in misuse by individuals unaware of their limitations. The manual has been organized in a stepwise style, so the user can develop a program for use of colorimetric detector tubes.