With a repertoire of more than 100 roles, Adam appeared with companies all over the world, from the Vienna State Opera to the Metropolitan Opera and from San Francisco and Chicago to Salzburg and Bayreuth (where he sang for 28 seasons). While he was known especially for his Strauss and Wagner — in particular for his portrayal of Hans Sachs in Der Meistersinger — he sang Verdi, Mozart and even Berio as well.
Alongside his opera career, Adam regularly performed in lieder recitals and choral and orchestral concerts; he was the bass soloist for many of the Bach recordings made in the 1960s and '70s by conductor Karl Richter and the Munich Bach Choir and Orchestra.
In 1972 Adam began another operatic career — as a director, staging works by Wagner, Mozart, Tchaikovsky and Strauss at the Semperoper, the Berlin Staatsoper and the Bavarian State Opera in Munich.