* Gossip Girl (8 pm/ET The CW)'''
The show takes a trip back to the totally awesome '80s as Serena's arrest prompts Lily to recall her wild and crazy youth. Flashbacks tell the tale of a teenage Lily (Brittany Snow) getting kicked out of boarding school and heading to L.A. to be with her family. No Doubt portray an '80s band and former Brat Packer Andrew McCarthy steps back into his Reagan-era duds to play Lily's dad. McCarthy won't be taking Molly Ringwald to prom, but Nate and Blair deal with their own prom problems back in the present day.
* House (8 pm/ET Fox)'''
Cuddy and the detoxed (for now) House got it on last week. Where can this go? "I don't think it's a passing thing," Lisa Edelstein said recently, just before a warning not to expect a "satisfying relationship." Thus teased, we move into tonight's season finale, in which we can also expect Cameron-Chase developments. Patients include a guy whose brain is at war with itself and a codger (Carl Reiner) that Cuddy foists on House. She's still his boss, remember, and he's not exactly Employee of the Month.
* The Big Bang Theory (8 pm/ET CBS)
Along with stellar writing and a perfect ensemble cast that has great chemistry - or would that be physics, in this case? - the series uses a stock sitcom device that keeps fans watching: a couple that everyone knows should be together, but who never quite get there. Fans know Penny and Leonard have "something," but Penny and Leonard don't see it clearly until tonight's second-season finale, in which Leonard prepares to leave for a three-month-long science trip.
Lucas dons a groom's tux for the second time, and hopefully he won't be left at the altar once again. Of course, last year his lingering love for Peyton was the reason Lindsey left him, and this year his betrothed is none other than Peyton herself, so things should go much more smoothly. Surprise guests are on hand for the nuptials, and Haley gets to play a special role in the ceremony. Meanwhile, Skills makes sure he doesn't lose sight of Jamie at this wedding, and Brooke keeps an eye on Nick Lachey.
* The Alzheimer's Project (7:30 pm/ET HBO)
Executive producer Maria Shriver, whose father, Sargent Shriver, has Alzheimer's, hosts the second installment of this vital documentary series about the degenerative brain disease. Based on Shriver's book What's Happening to Grandpa? this episode lets kids and teens share stories about their grandparents' varying stages of dementia, and offers advice for those who may experience fear, confusion or even anger at their loved ones' decline. More episodes of the series air tonight (8 pm/ET) and tomorrow (7 pm/ET).