In Buddhism upāya is a skillful means or method adopted in order to end suffering and lead others towards enlightenment. The manifestation of upāya can vary among the various schools of Buddhism, however. This thesis explores the role of upāya in the Shingon School of esoteric Buddhism in Japan. I argue that the seeming inconsistencies within Buddhist doctrines that appear in the Shingon School are best understood, and removed, when viewed through the lens of upāya. Kūkai, the founder of the Shingon School, was convinced of the superiority of his methods above all others. The Mandala of the Two Realms (Ryokai Mandala) and the ritual performed by the Shingon practitioner is central to Kūkai's methods. Clearly aware of his Buddhist predecessors, Kūkai builds on the philosophy of Nāgārjuna, who is recognized as a patriarch of the Shingon School. In this way we will see how the Mandala of the Two Realms communicates to us iconographically what Nāgārjuna communicates epistemologically through his Twofold Truth and explore what this has to do with upāya.