In the 1960's, Judd began to experiment with constructing solid geometric shapes. His work often described as Minimalism, his objects occupied three-dimensional space and thoroughly rejected illusionism.
His work is almost mathematically precise but he claims that the maths ingrained in his work is not important. He has little patience for those critics that claim his work (and that of other minimalists) has no meaning.
One of my favourite artists, his work focuses purely upon the space occupied by the works themselves, and the aesthetic qualities borne from them. I saw his 'stack' exhibited in the Pompidou Centre in Paris, which you can view a photo of here: