The reason for the divorce is the usual "irreconcilable differences," according to ET. Phoenix is seeking spousal support, as well as joint legal and physical custody of their sons, 12-year-old Indiana and 9-year-old Atticus.
They legally separated in November 2015, although they didn't announce their split until March 2016. (The photo above is from 2013.)
Phoenix did not accompany Affleck to the Oscars earlier this year, where he won Best Actor for "Manchester by the Sea." As you recall, old allegations of sexual misconduct dogged Affleck throughout awards season.
Summer is the youngest sibling of the Phoenix family -- River, Joaquin, and Liberty. As a child, she guest starred on TV shows including "Airwolf," "Murder, She Wrote" and "Growing Pains." Her films include "Dinner Rush," "The Believer" and "The Laramie Project."