Early work by Dingle and co-workers on the GaAs-(AlGa)As heterojunction established the fraction of the energy gap difference appearing in the conduction band to be 0.85. The experimental evidence and techniques which established this fraction and the subsequent evidence that has led to the questioning of its previous, almost universal, acceptance will be critically reviewed. Experimental evidence for the offset ratio in this system being dependent upon growth sequence will be discussed. Existing theoretical models will be discussed in the context of the above experimental work.