Watch HBO's "I Know This Much Is True" online tonight, and Mark Ruffalo will appear twice.

"I Know This Much Is True" is a six-episode limited series based on the best-selling novel by Wally Lamb, an Oprah Winfrey Book Club selection. Written and directed by Derek Cianfrance, who also wrote "Blue Valentine" and "Place Behind the Pines."

The story follows identical twin brothers Dominic and Thomas Birdseye and their trials and tribulations. Melissa Leo, Rosie O'Donnell, Archie Panjabi, and Imogen Poots also star.

Ruffalo, whose recent credits include playing Bruce Banner/Hulk in the "Avengers" films, is also an Academy Award-nominated actor for "Spotlight," "Foxcatcher" and "The Kids Are All Right." He also won a SAG for his role as a gay writer and activist in the HBO TV movie "The Normal Heart."

He served as executive producer on I Know This Much Is True, which HBO described as a "family saga" and "an epic story of betrayal, sacrifice, and forgiveness." The film's unflinchingly gritty narrative depicts Thomas' paranoid schizophrenia and Dominic's struggle to save his brother while dealing with his own failed marriage and the death of his young child.

Here's everything you need to watch I Know This Much Is True online:

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For U.S. viewers, "I Know This Much Is True" Episode 1 premieres tonight (Sunday, May 10) at 9 p.m. ET on HBO if they subscribe to a premium cable network. Watch it on cable or stream it live on HBO Go.

If you don't have cable, you can sign up for HBO Now for $14.99 per month.

HBO is also available as an add-on to most popular live TV streaming services, including Sling TV, Hulu with Live TV, and YouTube TV. HBO is also available a la carte on Apple TV Channels.

Good news for Brits, I Know This Much Is True can be seen on Sky Atlantic & NOW TV on Monday, May 11 at 9pm BST, the day after it airs in the US.

More good news for Canadians: I Know This Much Is True will air on Crave at the same date and time as in the US.

Yes, you're seeing twice as much Ruffalo:

As mentioned several times, Mark Ruffalo leads HBO's adaptation as twin brothers Dominic and Thomas Birdseye. He is joined by a talented cast of actors:

Other cast members for I Know This Much Is True include:

Reviews of I Know This Much Is True have praised how well the series, from the writing and direction by Cyanide France to the acting, especially Ruffalo's performance. praised how well done it is. However, critics also noted the dark and depressing nature of the story. The following is a roundup of the critiques.

AV Club "I Know This Much Is True may not be an enjoyable film, but it is an engaging one, thanks to the excellent performances of its impeccable cast."

Rolling Stone Magazine So relentlessly, almost proudly miserable at every turn, that after a while I began to see the project as an unexpected form of public service, not to mention the 79-minute finale, each hour of which was tough to get through. But each time I left, I left the world feeling a little better about the current state of my own life. And it was also in the midst of a pandemic quarantine.

The Hollywood Reporter: ". If the acting in I Know This Much Is True were not so terrific, the catharsis would probably not justify the exhausting length. "

The New York Times And while "I Know This Much Is True" pulls the audience in with its soap opera mechanics, smoothly downbeat atmosphere, and the merits of Ruffalo's performance, it promises more than it delivers.