The present paper surveys visual attention models, showing factors’ categorization. It also studies bottom-up models in comparison to top-to-down, spatial models compared to spatial-temporal ones, obvious attention against the hidden one, and space-based models against the object-based ones. It categorizes some challenging model issues, including biological calculations, correlation with the set of eye-movement data, as well as bottom-up and top-to-down topics, explaining each in details.