This is a study of Gustav Mahler's interpretative style as a conductor of the symphonic repertoire which focuses particularly on his instrumental Retuschen. His performance practice as a conductor is related to his own concert career, the interpretations of his close predecessors, contemporaries and successors, studies of his concert repertoire, his conducting technique and rehearsal methods. A catalogue is included of sixty sources from public and private collections in Europe and America comprising scores and orchestral parts of works by Bach, Beethoven, Bruckner, Mozart, Schubert, Schumann, Smetana and Wagner. Description, analysis and comparison of these sources is supplemented by contemporary accounts of Mahler's conducting. Both primary and secondary sources are employed to establish the relative importance of the scores and orchestral parts and to assign dates when they were used by Mahler. The individual discussion of the works is accompanied by analyses of Mahler's treatment of each of the instruments of the orchestra, including the extensively used E-flat clarinet. Mahler's response to acoustics, his changes of dynamic nuances, tempi, attitude to repeats, and cuts are all considered in detail. His performance practice and instrumental Retuschen in Hamburg, Vienna and New York are also compared.