Heidegger's middle and late writings are explored in light of a theme central to his thought: the ontological difference. After arriving at an adequate formulation of the ontological difference, Dr. Vail explores the implications of the Difference for human existence in all of its philosophical facets. The Difference must be understood in terms of the duality of revealing/concealing, and the expression of this duality is the truly radical element in Heidegger's thought. The author demonstrates areas of philosophical thought where this logic of revealing/concealing is important: the essence of man. technology, art, and language.