The life of German anarchist thinker Gustav Landauer (1870-1919) spans the same timeframe as the Empire, which was established one year after his birth and collapsed one year before his death. In the political and intellectual life of that period, Landauer emerges because of his opposition to both the Empire and to the movement then considered as the main alternative to the imperial organisation: the German social-democracy of Marxist allegiance. He worked with a thorough and careful approach on the main issues of his time, be they political, social, linguistic or philosophical. Compared to other authors of the German philosophical and political tradition, Landauer has been relatively little studied in Germany, and his writings (which include theoretical, journalistic, literary works as well as translations and a significant correspondence) are hardly known in France due to the limited amount of translations available. Although he has been often mentioned in his relationship to other authors, and sometimes for his political activism, the many literary, linguistic or historical approaches devoted to Landauer do not foster a complete understanding of his complex philosophical system. Compared to the other four prominent authors of classical anarchism (Proudhon, Bakunin, Kropotkin and Malatesta), Gustav Landauer is the missing thinker from philosophical studies in this field. This thesis will strive to renew the perspective on some key philosophical debates of that time, as well as to bring a new understanding of some major issues of political thought from the end of the 19th century to the 21st century.