Matthew Fox, Kiefer Sutherland Matthew Fox, Kiefer Sutherland

With Lost, 24 and Law & Order all leaving the air this spring, network TV has a lot of big shoes to refill. It's been an aggressive development season, and all the networks are hoping their new shows will bring them the success these three heavyweights once did. With that in mind, TVGuide.com examines the issues the networks face as they prepare to announce their fall lineups this week.

Fall TV Scorecard: Which shows are returning? Which aren't?

ABC

The Alphabet already has some bright spots in its lineup (its Wednesday comedy block, the one-two punch of Desperate Housewives and Brothers & Sisters on Sunday night), but with the loss of Lost and the lack of any real strength in the 10 p.m. hour (Castle being perhaps the exception), its drama pickups will be scrutinized.

Three promising pilots have already been green-lit: No Ordinary Family, starring The Shield's Michael Chiklis and Dexter's Julie Benz as the heads of a family with special powers; Body of Proof, which borrows Housewives' Dana Delany as a medical examiner; and Shonda Rhimes' Off the Map, which is kind of like Grey's Anatomy in the jungle.

CBS

Ratings aren't usually a problem for the Eye — they own Friday nights, continue to dominate with crime procedurals and solid reality (Survivor, The Amazing Race) throughout the week and have a strong Monday night comedy block. Rather, its problem has always been demographics, and its lineup is aging as quickly as its audience. The CSI franchise, though still strong, has logged declining ratings.

Freshman dramas The Good Wife and NCIS: Los Angeles have been promising additions to its lineup. Thus far, the network has been pretty quiet about new pickups, but with the cancellation of Three Rivers, and the rumored axings of Numb3rs, Cold Case and Miami Medical, it'll have some slots to fill.

FOX

The end of 24 leaves Fox without a dramatic tentpole. House and Bones are still going strong, Glee hit a high note in the ratings, and for now, it's hanging on to Fringe, Human Target and Lie to Me, despite mediocre ratings. Its Sunday-night animated shows continue to do well, but it hasn't fared as well with live-action comedies: It's already canceled 'Til Death, Brothers and Sons of Tucson after abysmal ratings.

Its most buzzed-about pilot, the Steven Spielberg-produced, time-travel dinosaur drama Terra Nova, is still looking for a star, as both Kyle Chandler and Kevin Bacon have reportedly turned down the role.

The wild card: Simon Cowell's The X-Factor, which Fox is launching in the fall of 2011 as an American Idol companion piece. It remains to be seen whether it will capture American audiences in the same way that Idol has.

Get the scoop on all the new fall shows

NBC

With the failure of Jay Leno's prime-time show and the cancellations of Heroes, Law & Order, Mercy and Trauma, NBC has a lot of holes to plug. An aggressive development season has led to some early pickups, though, many of which made the network's fall lineup.

CW

The network made a lot of early pickups of its strong-in-the-demo lineup, including breakout hit The Vampire Diaries, 90210, Gossip Girl, Supernatural and Smallville. Melrose Place is expected to be officially canceled any minute now, which leaves One Tree Hill and Life Unexpected still undecided. No word yet on any new-show pickups.

TBS/TNT

With the surprise announcement that Conan O'Brien will be launching a late-night show on TBS, plus the acquisition of the critically acclaimed Southland by TNT, these cable channels are suddenly contenders for a larger piece of the network audience. Burning question: Will TNT pick up Law & Order so it can have its record-breaking 21st season?