Sofia Vladimirovna Vassilieva is an American actress. Notable roles include portraying the children's book character Eloise in Eloise at the Plaza and Eloise at Christmastime, Ariel DuBois in the Emmy-winning TV series Medium, and teenage cancer patient Kate Fitzgerald in the 2009 film adaptation of My Sister's Keeper by Jodi Picoult.

Medium is an American supernatural drama television series created by Glenn Gordon Caron that originally aired on NBC for five seasons from January 3, 2005, to June 1, 2009, and on CBS for two more seasons from September 25, 2009, to January 21, 2011.