Batman is an ongoing comic book series featuring the DC Comics hero of the same name. The character first appeared in Detective Comics #27 (cover dated May 1939). Batman proved to be so popular that a self-titled ongoing comic book series began publication in the spring of 1940. It was first advertised in early April 1940, one month after the first appearance of his new sidekick, Robin, the Boy Wonder. Though the Batman comic book was initially launched as a quarterly publication, it later became a bimonthly series through the late 1950s, after which it became a monthly publication and has remained so since. The original series ended in 2011 and was relaunched with a new first issue. The character of Batman made his first appearance in the pages of Detective Comics #27 in May 1939. In Spring of 1940, Batman #1 was published and introduced new characters into Batman's pantheon, most notably those of Catwoman and Batman's eventual nemesis, the Joker.