And to Think That I Saw It on Mulberry Street is Theodor Seuss Geisel's first children's book. It was published under the pen name Dr. Seuss. First published by Vanguard Press in 1937, the story follows a boy named Marco, who describes a parade of imaginary people and vehicles traveling along a road, Mulberry Street, in an elaborate fantasy story he dreams up to tell his father at the end of his walk. However, when he arrives home he decides instead to tell his father what he actually saw—a simple horse and wagon.

Geisel conceived the core of the book aboard a ship in 1936, returning from a European vacation with his wife. The rhythm of the ship's engines captivated him and inspired the book's signature lines:

At least 20 publishers rejected the book before Geisel ran into an old college classmate, who had just become juvenile editor at Vanguard Press. Vanguard agreed to publish the book, and it met with high praise from critics upon release, though sales were not as impressive. Later analyses of the book have focused on its connections to Geisel's childhood; the street of the title is probably named after a street in Geisel's hometown of Springfield, Massachusetts. Geisel returned to fictionalized versions of Springfield in later books, and Marco appeared again in 1947 in the Dr. Seuss book McElligot's Pool.