Book excerpt: The eleventh Symposium of the International Research Group on Colour Vision Deficiencies (IRGCVD) was held 20-23 June 1991 in Sydney, Aus tralia, ably hosted by local organizer Stephen Dain. A total of 35 talks and 10 posters were presented. Papers based on 37 of these presentations are included here, in Colour Vision Deficiencies XI. The scientific program featured sessions on three special topics, with each topic highlighted by an invited speaker. The opening session on the Genetics of congenital colour vision deficiencies began with a superb invited lecture by Charles Weitz about his pioneering work on the molecular genetics of tritanopia. The session on the second special topic, Spatial aspects of colour vision, began with the launching of a new IRGCVD tradition, as 1991 Verriest Memorial Award recipient Harry Sperling presented the first Ver riest Memorial Lecture on his recent studies of spatial discrimination of heterochromatic stimuli. Dr. Sperling reported new evidence that certain asymmetries in red-green opponent colour vision can be explained by the spatial organization of colour-opponent retinal neurons. In the third special session, on Occupational aspects of colour vision, Barry Cole took the audi ence on a fascinating tour of the historical development of colour vision standards in his invited lecture entitled 'Does defective colour vision really matter?'. In addition to the three special topics, many interesting presentations were given in topical sessions on Variations in normal colour vision, Acquired colour vision deficiencies and Colour vision tests and testing methods.