This volume offers the essential theoretical thought of the Austro-Marxist thinkers Otto Bauer, Max Adler, Karl Renner, Friedrich Adler, Rudolf Hilferding, and Otto Neurath over the span of their Austrian Social-Democratic careers, from the decades before World War I until the mid-1930s. Austro-Marxist theoretical perspectives were conceived as social scientific tools for the issues that faced the development of socialism in their time. The relevance of their thought for the contemporary world inheres in this understanding.