Martin Heidegger (1889–1976) is widely considered to be the most famous, influential and controversial philosopher of the twentieth century. His writings are also among the most formidable. The fundamental concepts of his thought are for many the source of both fascination and frustration. Yet any student of philosophy – or of contemporary thought in general – needs to become acquainted with Heidegger's main ideas. This book is designed to facilitate this process. Each chapter introduces and explains a key concept – or a cluster of closely related concepts – in Heidegger's thought. Together, the chapters cover the full range of his path of thought in its early, middle and later periods.