I find it curious that composers such as Benjamin Britten (1913-76), Dmitri Shostakovich (1906-76), and Aaron Copland (1900-1990) were omitted from the list of "some of the principal composers of the Modern era", which includes names like Lukas Foss, Edison Denisov, and others, who's works have been far less influential, than the three, whom I've mentioned. Which is not to say that the music of Foss, and others should not be better known, but I fail to see how the first three composers can not be considred "principal".