1 INTRODUCTION The scope and applications of odor measurement and are sometimes confused by control uses of the word odor. In the past, odor has been used to mean either the perception of smell referring to the sensation, or which is smelled, referring to the stimulus. To eliminate confusion, odor is best used only for the former meaning, and odorant should be defined as any odorous substance. odor intensity, then, is the magnitude of the olfactory sensation produced on exposure to an odorant. control that can be used to describe any process that makes olfactory experiences more acceptable to people. perceptual route to this objective is usually, but not always, the reduction of odor intensity. An alternative route is the change of odor quality in some way that is considered to be an improvement When the reduction of odor intensity is accomplished by removal of odorant from the atmosphere, the process is equivalent to gas and vapor abatement, or to air cleaning, but with some special considerations. These include(I) problems related to the need to attain very low concentrations, often approaching threshold levels, (2) uncertainties with regard to the reliability of sensory or chemical analyses(see Section 4), and(3) difficulties as- sociated with diffuse or sporadic sources. When an odorous atmosphere is improved by the addition of another(usually pleasant) odorant under conditions in which chemical reactions are not involved, the process is called odor modification, referring to modification of the odor perception, not of the odorant. These and other methods of odor control are considered in some detail in Section 6