A comprehensive discussion of the various methods of analysis for dissolved oxygen in natural and waste waters is presented. Special emphasis is given to pointing out the applicability and limitations of each method. Several titrimetric methods and micro techniques are described. Manometric, gas-exchange, gas chromatographic, radiometric, and polarographic techniques and galvanic and membrane electrodes are among the instrumental methods discussed. In situ measurements of the activity of molecular oxygen are claimed to be more significant than concentration measurements. Examples of laboratory and field studies are given.