Harold Sidney Harmsworth, 1st Viscount Rothermere, was a leading British newspaper proprietor who owned Associated Newspapers Ltd. He is best known, like his brother Alfred Harmsworth, later Viscount Northcliffe, for the development of the Daily Mail and the Daily Mirror. Rothermere was a pioneer of popular tabloid journalism, and an enthusiastic proponent of closer links between the UK and Nazi Germany of which he was a prominent British admirer.

Alfred Charles William Harmsworth, 1st Viscount Northcliffe, was a British newspaper and publishing magnate. As owner of the Daily Mail and the Daily Mirror, he was an early developer of popular journalism, and he exercised vast influence over British popular opinion during the Edwardian era. Lord Beaverbrook said he was "the greatest figure who ever strode down Fleet Street." About the beginning of the 20th century there were increasing attempts to develop popular journalism intended for the working class and tending to emphasize sensational topics. Harmsworth was the main innovator. He said, "News is something someone wants to suppress. Everything else is advertising."