Tonight’s episode of “ER” (9 p.m., NBC) will be that drama’s 300th installment. I’m not sure how much these milestones mean outside of a small circle of producers, performers and writers, but the number does put the series in some perspective.

“ER” has been around longer than any scripted series except “Law & Order” and “The Simpsons.” When it first aired in 1994, it had the NBC “must-see” lineup as wind in its sails, including “Mad About You,” “Friends” and “Seinfeld.” Of course, viewers had to sit through “Madam of the People,” too.

“ER” is old enough to belong to another TV era, an epoch when “Blossom,” “M.A.N.T.I.S” and “The Five Mrs. Buchanans” walked the earth.

It would take a very long column, or an obsessive blog, to list the number of cast changes or the number of characters who have grown ill, become addicted or died during the drama’s 13 years. Or to calculate the number of times the hospital has been under siege, or the number of bus, car, plane or helicopter crashes that have flooded the emergency room with patients and the show with guest stars. And let’s not even try to calculate the number of tracking shots. Long before “The West Wing” became the most frantic production on television, the cameras on “ER” were whipping around the emergency room.

It’s no secret that “ER,” once among the most popular shows in prime time is not as highly rated as it was. It’s been eclipsed as a medical drama by “Grey’s Anatomy” and its spin-off, “Private Practice,” shows closer to “Desperate Housewives” in their near farcical fixation on sex and melodrama. It remains to be seen whether either of those shows will be around for its 300th episode.

Meanwhile, “ER” keeps it on the serious side. In tonight’s episode, Abby and Kovac return to Croatia to deal with a death in the family.

¢ George Clooney, the most famous former cast member of “ER,” narrates “Sand and Sorrow” (7 p.m., HBO), a documentary look at the slaughter in Darfur, the poor western section of Sudan where non-Arab residents have been murdered, raped and scattered by forces controlled by the Khartoum regime.

¢ “CNN Heroes” (8 p.m., CNN) honors “everyday people” for extraordinary deeds at a ceremony broadcast live from New York’s Museum of Natural History. Look for performances from Wyclef Jean with Norah Jones, Mary J. Blige and Sheryl Crow.

Tonight’s other highlights

¢ After the warden’s double-cross, Earl opts for an early exit on “My Name is Earl” (7 p.m., NBC).

¢ A big double date on “Ugly Betty” (7 p.m., ABC).

¢ Jack’s secret lover (Edie Falco) wants to go public on “30 Rock” (7:30 p.m., NBC).

¢ Michael addresses Ryan’s business class on “The Office” (8 p.m., NBC).

¢ Warrick’s past demons get in the way of a case on “CSI” (8 p.m., CBS).

¢ Meredith scrambles to save the life of a paramedic on “Grey’s Anatomy” (8 p.m., ABC).

¢ A woman (Linda Hamilton) loses everything in her divorce, just in time for the holidays, in the 2006 made-for-cable bummer “Home by Christmas” (8 p.m., Lifetime).

¢ A troubled ex-con vanishes “Without a Trace” (9 p.m., CBS).

¢ In a time of anxiety and change, Barbara Walters considers Victoria Beckham among “The 10 Most Fascinating People of 2007” (9 p.m., ABC).

Cult choice

Kathy Ireland plays Santa’s daughter in two made-for-cable holiday romantic comedies “Once Upon A Christmas” (6 p.m., Family) and “Twice Upon A Christmas” (8 p.m., Family) from 2000 and 2001.