Mark and Addison in a photo from "Blink." A crossover event is totally in effect as Grey's Anatomy finally returns from its hiatus on January 14, 2010.

Addison and Callie go to church on Grey's Anatomy, as seen in this photo from "Before and After."

Derek, Addison and Callie in "Blink." Grey's Anatomy finally returns from its hiatus on January 14, 2010.

Addison (Kate Walsh) is back on Grey's Anatomy in this photo from "Before and After," from Season Five.

Sometimes, the Grey's Anatomy cast is not enough. Someone from Private Practice needs to be brought in for expertise.

Another photo of Addison and Callie in church on Grey's Anatomy in the Season Five episode, "Before and After."

Can it be done? Are the doctors at Seattle Grace up to the task? They are, we think, but they will have their hands full.

Mark and Addison used to have a thing. Now the only thing they are worried about is saving her brother's life.

These best buds may be seeing more of each other.

Addison and Callie were good friends at one point, and are now reunited, however briefly, on Grey's Anatomy in this photo from "Before and After."

A photo from the May 1, 2008 episode of Grey's Anatomy, entitled "Piece of My Heart." Addison Montgomery is back at Seattle Grace!

A nice promotional photo from the May 1, 2008 episode of Grey's Anatomy, which is being called "Piece of My Heart." Dr. Addison Montgomery (Kate Walsh) is back in Seattle for one episode only. Too bad she can't stay longer!