The meaning of 72 odour qualities is reported in a Glossary. The information was derived from three main studies. Replies to two questionnaires provide information on frequency of understanding, and the meaning of the qualities understood, expressed in the respondents' own words. The odour stimuli used in the experimental investigations are specified and the rationale for their selection is outlined. In Expt. I, 53 odour stimuli were characterized spontaneously, apart from the aid of a Guidance List. The results are then expressed in terms of the frequency of use of the main qualities. In Expt. II the characterization of 45 odour stimuli was directed by the use of a five-point rating scale with 44 carefully chosen qualities. Data are presented separately for experienced and inexperienced subjects. The experimental results relating to each quality are summarized. Some of the wider implications of these studies are discussed.