Writing in the 1880s, Galton recognized that some healthy individuals lack visual imagery. This phenomenon has been relatively neglected since then. In 2015 we coined the term “aphantasia” to describe the lack of the mind’s eye, reporting on twenty-one individuals with a lifelong lack of imagery. Since then we have been contacted by many thousands of people lying at both the aphantasic and the hyperphantasic extremes of the vividness spectrum. Preliminary evidence suggests that lifelong aphantasia is associated variably with prosopagnosia and reduction in autobiographical memory; hyperphantasia is associated with synaesthesia. Over 50 percent of people with aphantasia report visual dreaming. In around 50 percent of our aphantasic participants, all modalities of imagery are affected, while in others some modalities of imagery are preserved. Aphantasia often runs in families. Functional imaging studies of imagery suggest a range of hypotheses for the neural correlates of aphantasia; the few functional imaging studies specifically examining imagery vividness point to positive correlations with brain activity in higher visual and memory-related areas. The study of aphantasia reminds us how easily invisible differences can escape detection. Visualization is only one of many ways of representing things in their absence, and individuals lacking visual imagery can be highly imaginative.