Theodor Seuss Geisel ( (listen); March 2, 1904 – September 24, 1991) was an American children's author and cartoonist. He is known for his work writing and illustrating more than 60 books under the pen name Dr. Seuss (). His work includes many of the most popular children's books of all time, selling over 600 million copies and being translated into more than 20 languages by the time of his death.

Geisel adopted the name "Dr. Seuss" as an undergraduate at Dartmouth College and as a graduate student at Lincoln College, Oxford. He left Oxford in 1927 to begin his career as an illustrator and cartoonist for Vanity Fair, Life and various other publications. He also worked as an illustrator for advertising campaigns, most notably for FLIT and Standard Oil, and as a political cartoonist for the New York newspaper PM. He published his first children's book And to Think That I Saw It on Mulberry Street in 1937. During World War II, he took a brief hiatus from children's literature to illustrate political cartoons, and he also worked in the animation and film department of the United States Army where he wrote, produced or animated many productions including Design for Death, which later won the 1947 Academy Award for Best Documentary Feature Film.

After the war, Geisel returned to writing children's books, writing acclaimed works such as If I Ran the Zoo (1950), Horton Hears a Who! (1955), The Cat in the Hat (1957), How the Grinch Stole Christmas! (1957), Green Eggs and Ham (1960), One Fish, Two Fish, Red Fish, Blue Fish (1960), The Sneetches and Other Stories (1961), The Lorax (1971), The Butter Battle Book (1984), and Oh, the Places You'll Go! (1990). He published over 60 books during his career, which have spawned numerous adaptations, including 11 television specials, five feature films, a Broadway musical, and four television series.

Geisel won the Lewis Carroll Shelf Award in 1958 for Horton Hatches the Egg and again in 1961 for And to Think That I Saw It on Mulberry Street. He received the Regina Medal award from the Catholic Library Association in 1982. Geisel's birthday, March 2, has been adopted as the annual date for National Read Across America Day, an initiative on reading created by the National Education Association. He also received two Primetime Emmy Awards for Outstanding Children's Special for Halloween Is Grinch Night (1978) and Outstanding Animated Program for The Grinch Grinches the Cat in the Hat (1982).