The Teaching of Reality is, as far as we know, the first translation into English of Tattvopadeśa, a work attributed to the great spiritual teacher Śankara. The text reveals the full significance of the great sentence 'You are That' (tat twam asi), which is said to convey the essence of the entire teaching of the Upanishads and to be the key to Advaita, the philosophy of non-duality. The reader is led through a systematic process enabling the real Self to be realised in practice.