The substance of this volume by the man regarded by many as the foremost existentialist of our time constituted a course of lectures delivered to students at Freiburg University in 1950-51. The style shows the impact of the spoken word, an outcome that Heidegger himself placed high among the significant formative functions of human beings. The series can be seen as marking a turn in the author's thought, as he passed from the questions that preoccupied him during the years of Nazism in Germany to the questions arising in the past decade and a half. Thinking is not having an opinion, or notion, or idea; it is not ratiocination; it is not conceptual or systematic. Rather it is a response of men to a call that issues from the nature of things -- from Being. As such, it cannot be willed or wished, although we can prepare ourselves to hear the call. But the meaning of thinking can be known only when we ourselves try to think. This volume, the twentieth in the Religious Perspectives Series, will be of primary interest to scholars and students of Heidegger and/or existentialism. But its lively and lucid style can recommend it to a broader range of serious readers.