Although recent Japanese debates over hongaku thought have attracted considerable attention in the West, very few studies of actual Tendai hongaku texts have appeared in Western languages. In this study one text, the Kanko ruiju, is examined and placed in a historical context to demonstrate that it is a serious attempt by medieval Japanese scholars to interpret Chih-i's Mo-ho chih-kuan. Particular attention is paid to issues of how Zen models of lineage might have contributed to the text’s formulation and how monastic debate allowed monks to select certain passages for intensive scrutiny. The use of the text in debates helps explain the appearance of new classification systems of doctrine, and the variety of positions on doctrine and practice advanced in the Kanko ruiju.