Decoding the psychological dimensions of human odor perception has long been a central issue of olfactory research. Odor scientists as well as fragrance professionals have tried to establish comprehensive standards for the description, measurement, and prediction of odor quality characteristics. As odor percepts could not be linked to a few measurable physicochemical features of odorous compounds or physiological characteristics of the olfactory system, odor qualities have often been assessed by perception-based ratings. Although they have been applied for more than 5 decades, these psychological approaches have not yielded a comprehensive or generally accepted classification system yet. We assumed that design and methodology of these studies have largely prevented the development of unbiased odor arrangements. To address this issue, we reviewed 28 perception-based classification studies and found that their outcome has been largely determined by 4 influencing factors: 1) interindividual differences in perceptual and verbal abilities of subjects, 2) stimuli characteristics, 3) approaches of data collection, and 4) methods of data analysis. We discuss the effects of each factor in detail and illustrate how odor systems have reflected perceptual qualities as well as the conditions under which these have been assessed.