The different psychophysical studies carried out on the qualitative properties of simple and complex mixtures are reviewed. Many of the early studies conducted on binary mixtures have been reprocessed and compared by using similar representations and data treatments. Results confirm that the intensity ratio of the mixture constituents is the main driver of quality, so that the most intense compound tends to dominate the overall odour of the mixture. In contrast to what was previously thought, in most binary mixtures one of the components is more powerful than the other, dominating the overall odour of the mixture in a larger range of mixture compositions. In some but not all subsets of odorants, a clear hierarchy in their relative dominance has been observed. Pleasantness, complexity or freshness arise as major but not unique factors determining the order of dominance. Steven's coefficients could also have an influence, while polarity and temporal processing rates seem to have no influence. Competitive interactions at receptor level seem to play a limited role. In some aroma pairs, a small change in odour composition (26%) provokes the shift in odour dominance, while in others a 500% intensity change may be required for the shift. There is sound evidence that one or several odour descriptors of minor non‐dominant odour components can be detected in the odour of the mixture. In more complex mixtures, results support the fact that some odorants in specific odour backgrounds are detected more easily than others. The most frequent qualitative outcome of a complex mixture will be an average of the odour qualitative descriptors of the odour chemicals present in the mixture, except when the mixture contains within its constituents a series of odour chemicals at specific intensity ratios able to trigger a configurational recognition process ending with the recognition of a different odour object. Key challenges for the modelling of the aroma properties of complex systems are enunciated.