Genshin (源信; 942–1017), also known as Eshin sōzu (恵心僧都) , is one of the most prominent masters of Japanese Tendai Buddhism. This study focuses on one of his seminal works, the Ichijō yōketsu 一乗要決 (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 Ichijō yōketsu and the Ōjōyōshū 往生要集 (The Essentials of Birth in the Pure Land), were understood in the Buddhist milieu of the Japanese Middle Ages. By shedding light how the Ichijō yōketsu was quoted in connection to the sangoku mappō 三国末法 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 sōzu den 源信僧都伝 (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.