Genshin (¿¿; 942-1017), also known as Eshin sozu (¿¿¿¿) , is one of the most prominent masters of Japanese Tendai Buddhism. This study focuses on one of his seminal works, the Ichijo yoketsu ¿¿¿¿ (Determining the Essentials of the One-vehicle), and evaluates its influence on subsequent representatives of the tradition in the medieval period. It elucidates how Genshin and his most significant writings, i.e. the Ichijo yoketsu and the Ojoyoshu ¿¿¿¿ (The Essentials of Birth in the Pure Land), were understood in the Buddhist milieu of the Japanese Middle Ages. By shedding light how the Ichijo yoketsu was quoted in connection to the sangoku mappo ¿¿¿¿ paradigm ("decline of the dharma throughout the three countries" of India, China, and Japan), the present book contributes original and relevant insights into Japanese Buddhism. It primarily takes an intertextual approach, which is carried out with the help of digital resources and tools. Yet another feature is the first English-language translation of the Genshin sozu den ¿¿¿¿¿ (Biography of the General Vicar Genshin), the oldest full-length biography of Genshin. While the work presents a wealth of new knowledge and hitherto unexplored angles, it does not intend to conclude its matter. Rather, it aims at inspiring further interest in the study of Genshin and his many unexplored works.