Roger Ebert, the popular film critic and television co-host who along with his fellow reviewer and sometime sparring partner Gene Siskel could lift or sink the fortunes of a movie with their trademark thumbs up or thumbs down, died on Thursday in Chicago. He was 70.

His death was announced by The Chicago Sun-Times, where he had worked for more than 40 years. No cause was specified, but he had suffered from cancer and related health problems since 2002.

It would not be a stretch to say that Mr. Ebert was the best-known film reviewer of his generation, and one of the most trusted. The force and grace of his opinions propelled film criticism into the mainstream of American culture. Not only did he advise moviegoers about what to see, but also how to think about what they saw.

President Obama reacted to Mr. Ebert’s death with a statement that said, in part: “For a generation of Americans — especially Chicagoans — Roger was the movies. When he didn’t like a film, he was honest; when he did, he was effusive — capturing the unique power of the movies to take us somewhere magical.”