This article is from the ER FAQ, by Rose Cooper firstname.lastname@example.org with numerous contributions by others.
The troubled marriage of Mark and Jennifer (or Jenn) Greene
hit many bumps on the way down.
During the first season, Mark drew the wrath of his wife by
accepting an attending position at the hospital without consulting
her first. Jenn had her eye on a similarly juicy position at a law
firm in Milwaukee. Jenn took her position as well, and this caused
the two to begin spending time apart. Eventually, Mark compromised
by regularly commuting via train from Milwaukee to Chicago, while
their young daughter Rachel lived in Milwaukee with Jenn.
However, in the episode "Home" , Jenn and Rachel were in
an auto accident on the way home from an office Christmas party.
Only Jenn was hurt, and then not seriously, but when he visited them
in the hospital soon after, Mark discovered that the driver of the
car was Craig Simon, one of Jenn`s fellow law clerks. Jenn admitted
that she`d been having an affair with Craig.
Since then, Mark and Jenn have been officially quits. As of
the end of the second season, their divorce is more or less final,
and they`ve agreed to share custody of Rachel. In addition, Jenn has
announced that she and Craig plan to marry, which has probably
happened by now.