Twelve years on television is an eternity. Marg Helgenberger will leave “CSI” (9 p.m., CBS) after two more episodes, beginning tonight. Her character, Catherine Willows, has been part of the forensics series since it debuted in 2000, outlasting male leads William Petersen and Laurence Fishburne.

”CSI” has long been compared to “Law & Order,” another long-running procedural franchise that spawned successful spinoffs and all but dominated its network’s schedule. But to many, “CSI” will always be associated with Grissom and Willows. Unlike “Law & Order,” it has been affected by its revolving door of lead characters. And it’s highly doubtful that “CSI” will run as long as “Law & Order.”

Helgenberger will also appear on “Late Show With David Letterman” tonight.

• Now in its 11th season, “American Idol” (7 p.m., Fox) shows little sign of flagging. To the surprise of many, it has changed personnel without missing a beat. It has persevered, even thrived in spite of Paula Abdul’s and Simon Cowell’s departures. In fact, Cowell’s “X Factor” was received with less than rapturous reviews and ratings, while “Idol’s” new judges, Steven Tyler and Jennifer Lopez, adjusted quite naturally to their roles.

Competing programs, including “The Voice,” ”America’s Got Talent” and “X Factor,” have never approached “Idol” in audience size. But their combined presence could contribute to a sense of talent show fatigue. And that could eventually contribute to the decline of “Idol.” On the other hand, too much of a good (or a bad) thing is rarely a problem in reality television.

• Far from the field of singing competition, “Greatest Tank Battles” (9 p.m., Military) spends 10 weekly episodes recalling mechanized warfare from World War I onward. It’s nice to see that somebody’s still broadcasting history programming. Last year, the History Channel got the highest ratings in its history by ignoring it.

• Speaking of history, “NOVA” (8 p.m., PBS) presents “3-D Spies of WWII.” It recalls how photo and optics experts from British universities and Hollywood studios compiled thousands of photographs to create 3-D images of potential targets for Allied bombers.

• S. Epatha Merkerson (“Law & Order”) hosts “Find Our Missing” (9 p.m., TVOne), documenting missing persons cases involving black men, women and children, who are generally ignored by the news media.

Tonight’s other highlights

• Zooey Deschanel stars in the 2009 romance “(500) Days of Summer” (7 p.m., E!).

• Twin brides plus one murder doesn’t add up on “Harry’s Law” (8 p.m., NBC). Jean Smart guest-stars.

• Harry Connick Jr. guest-stars on “Law & Order: SVU” (9 p.m., NBC).

• Daniel’s big proposal on “Revenge” (9 p.m., ABC).

• “True Life: I Have Diabetes” (10 p.m., MTV).