This series consists chiefly of incoming letters, arranged alphabetically by name, with a small amount of outgoing carbons interfiled. While most correspondence dates from Weigl's years in the United States, a significant number of earlier letters are also present. Much of the correspondence concerns Weigl's music, collaboration with performers, and, particularly in the United States, his teaching positions, efforts to secure performances of his music, and specific performances. Letters dating from the period of the Weigl family's emigration document Karl and Vally's search for legal sponsorship and employment, including correspondence with a network of former students and other musicians, philanthropists, and refugee organizations. Also dating from this period are a large number of letters answering an invitation to a private concert of Weigl's music given in New York City in April, 1939.