Mark Snow, a Juilliard-trained soundtrack composer who earned 15 Emmy Award nominations, including one for his eerily astral opening theme to “The X-Files,” a 1990s answer to the timeless “Twilight Zone” theme and the basis of a surprising dance hit in Europe, died on July 4 at his home in Washington, Conn. He was 78.

The cause was myelodysplastic syndrome, a rare form of blood cancer, his son-in-law Peter Ferland said.

Over an extraordinarily prolific five-decade career, during which he tallied more than 250 film and television credits, Mr. Snow excelled in a field that comes with built-in creative challenges.

“Some producers describe their musical idea as ‘fast but slow,’” he said in a 2000 interview with Film & Video magazine. “The director might say he wants to hear music that’s ‘blue with a hint of green.’ Now, no one really knows what those terms mean. That’s a big part of my job, interpreting the search for a project’s musical voice.”

Mr. Snow provided music for 90 episodes of “Hart to Hart,” which starred Robert Wagner and Stefanie Powers as a jet-setting couple who double as amateur sleuths, and 40 episodes of “Falcon Crest,” the 1980s prime-time soap opera.