Stephen King had long believed a television adaptation of It would never happen, since TV censorship rules at the time forbade depicting children in serious jeopardy, a major theme of the novel.

George A. Romero was originally signed to direct It when it was planned as an eight-to-ten-hour miniseries set in small-town Massachusetts rather than Derry, but he left due to a scheduling conflict with his 1990 remake of Night of the Living Dead, and the project was later condensed first to three parts and then to a two-hour, two-part miniseries once Tommy Lee Wallace signed on.

Writer Lawrence D. Cohen received King's manuscript for It in 1986, before the novel was even published, and his early script drafts abandoned the book's flashback structure until director Tommy Lee Wallace rewrote the script himself, reinstating the alternating child/adult timeline.

Malcolm McDowell, Roddy McDowall, and Alice Cooper were all considered for the role of Pennywise before director Tommy Lee Wallace insisted on casting Tim Curry, despite crew doubts since Curry was best known for comic roles like The Rocky Horror Picture Show and Clue.

Jim Carrey was originally considered for the role of Eddie Kaspbrak before Dennis Christopher was cast, and David Alan Grier was initially considered for Mike Hanlon before Tim Reid got the part.

John Ritter was a huge Stephen King fan who had made multiple failed attempts to appear in a King adaptation, even offering to 'play the girl,' and he finally landed the role of Ben Hanscom after George Romero's exit from the project freed up scheduling room.

Tim Curry was originally meant to play all of It's other forms in addition to Pennywise, but the production decided to cast different actors for the other incarnations to save time on makeup changes.

Because of his extensive makeup, Tim Curry had the earliest call time of any actor and required at least three hours each day just to remove his costume and makeup; he reportedly stayed in character between takes, chain-smoking and baring his sharp teeth to unnerve the child actors and make their fear more real.

It was filmed over three months in Vancouver, British Columbia, with a budget of $12 million, double the usual television movie budget, using locations such as Stanley Park, Beaver Lake, and the Buntzen Lake Hydro Plant, which stood in for the sewer plant.

The Chinese restaurant fortune cookie scene was shot in a single take without telling the actors what would happen, so their shocked reactions would be genuine.

Most of the special effects were achieved practically rather than digitally, including puppeteers animating the fortune cookies and replacement animation (similar to stop motion) used for scenes like Pennywise emerging from the drain or killing Belch in the sewers.

Pennywise's makeup design was based on Lon Chaney's look in The Phantom of the Opera (1925) stylized into a clown, and used PAX paint instead of traditional clown makeup to give him a 'living cartoon' appearance; Curry wore two sets of sharp teeth, a smaller flexible set for talking and a larger, more horrifying set for close-ups.

Despite mixed pre-airing reviews and competition from news coverage of President George H. W. Bush's foreign trips, It became ABC's biggest success of 1990, drawing a total of 30 million viewers across its two parts.

Twin Peaks was frequently referenced by the cast and crew during filming, as its success showed that unconventional television could still succeed, influencing Tommy Lee Wallace's approach to making It feel different and strange.