The Marvel Revolution (late 1950s to 1972)
Marvel's main rival, DC comics, successfully remade the Justice Society of America under Gardner Fox and Mike Sekowsky. They created the Justice League based on the Justice Society, and it became highly successful, prompting Martin Goodman to ask Stan Lee to create their own superhero team. Lee was actually thinking about quitting his job at Timely Comics because he was unsatisfied with the comic books he was writing there. He was unable to create or experiment with the comics he wrote because Goodman was comfortable with what they were doing then. When Goodman came to Lee with the idea for a new superhero team, Lee, encouraged by his wife Joan, decided to create them the way he wanted to.
With nothing holding him down, Lee created the Fantastic Four in collaboration with Jack Kirby (a comic book artist) in 1961. The creation of the Fantastic Four revolutionized the comic industry in many ways. For one thing, Lee created his team with the goal of "making them real characters in the real world." To this end, the four members of the team lived in New York City, while most superheroes at the time lived in fictional cities. With the goal of making his superheroes more realistic, Lee created his superheroes with flaws. It was this technique that changed the comic book industry. Suddenly, people could relate to the superheroes they loved and admired. They could see themselves in their heroes' pitfalls and connect with them on a more personal level. Superheroes went from perfect beings to normal beings who struggled with the same things that common men did. Lee achieved a great breakthrough with his new 2-dimensional characters, and his characters and their personal problems paved the future of comic books.
Fun Fact: Lee actually created the Fantastic Four without costumes, as he saw no need for superheroes to hide their talents. When their debut comic was published, Lee got a lot of fanmail that said his new superhero team was great, but that readers would only keep reading on one condition- Lee give his superheroes costumes. So Lee gave his fans what they wanted and created costumes for his groundbreaking new superhero team.