Stan Lee, one of the masterminds of the Marvel Comics universe, died at age 95.

Lee was the creator of dozens of now-iconic comic-book heroes, including Spider-Man, X-Men, the Avengers, the Fantastic Four and Black Panther. All of those characters have since been adapted into hugely popular.

Lee’s work creating characters for Marvel spanned the 1960s and into the early 1970s, but Lee was still involved with the production of the films based on his comics, and regularly made brief, tongue-in-cheek cameos within the movies.

In an essay remembering Lee posted Monday afternoon, Marvel and Disney said they “salute the life and career of Stan Lee and offer their undying gratitude for his unmatchable accomplishments within their halls. Every time you open a Marvel comic, Stan will be there.”

In a tweet Monday, Disney CEO Robert Iger said “Stan had the power to inspire, to entertain, and to connect. The scale of his imagination was only exceeded by the size of his heart.”