The Defenders (originally consisting of the Hulk, Doctor Strange, and the Sub-Mariner) first teamed together in Marvel Feature #1, 1971. This grouping proved popular enough to warrant the characters getting their own title, and soon they were joined by such heroes as the Silver Surfer, Nighthawk, and the Valkyrie.
The Defenders was a popular title throughout the 1970's, not only for its characters, but also because of the quality of the stories: writer Steve Gerber took readers on a wild ride featuring a murderous elf, a group of weird villains called the Headmen, a star-spanning religious cult, the revival of a once-laughable villain called Plant-Man, and more. The book was always interesting, and, thanks largely to the work of artist Sal Buscema, was always fun to look at.
The book evolved over time, so that eventually none of the original lineup remained. By the mid-1980's, the title reached a low point in story and art. One issue even had 1950's girl-comic heroine Patsy Walker revealed to be the daughter of Satan himself!