This is a transnational and bilingual investigation of the cross-fertilisation of mystical religiosity and modern poetical imagination in the works of the Bengali poet Rabindranath Tagore and the Irish poet W. B. Yeats. The book demonstrates how their commitments to transnational mysticism deeply form and inform the modernist literary projects of these poets as well as their understanding of cultural modernity. Although its primary interest lies in their poetry and poetics, the monograph also includes some of their relevant prose works. This study begins with a close look at and around the phase of 1912-1913, when Yeats and Tagore met over the collection of the latter's English translations of his spiritual verses, Gitanjali, and took mutual interests in each other's works and cultural significances. The monograph then expands on both sides of that phase, selectively covering the whole career of the poets in its exploration of their parallel mystic-modern cultural-poetical projects.