The relationship between the information the visual system gathers at its interface with the outside world and the perceptual reality of four primary hues and the resulting hue circle has been and remains of interest since the second half of the 19th century. Considerable effort has been expended over many decades in determining statistical mean choices of color stimuli that for individuals represent unique hues in given conditions of viewing. In this article, historical and recent data are used to show the present state of knowledge. To have a means of comparison, in this article spectral and display unit stimuli are converted to Munsell hues on basis of dominant or complementary wavelengths of highest chroma data. Relatively good agreement was found between spectral and color sample data, with display data being significantly different in case of unique green. The issue of significant interobserver variability found in all data and its implications are highlighted. Problems with generating simple psychophysical models, particularly related to unique red, are discussed and a brief survey of the current knowledge on neural processing that may lead to unique hue perception is presented. Data still required for a meaningful definition of mean unique hue stimulus data are listed. (c) 2013 Wiley Periodicals, Inc. Col Res Appl, 39, 279-287, 2014; Published Online 20 March 2013 in Wiley Online Library (). DOI 10.1002/col.21793