Overall, in the air pollution control field, odor concentration and intensity as well as hedonic rating have been well studied to the point where some level of standardization is being developed or is already in place. However, there has been no standardization with respect to odor quality characterization. There is now sufficient understanding of the types of odorous compounds that can arise from wastewater treatment processes to develop an odor classification scheme. This article presents the first wastewater odor wheel or classification scheme that should form the foundation for the evolution of odor quality data reporting with links to chemical causes.