13 longest-running scripted TV shows of all time

Excluding anthology series from the early years of television that featured different stories and characters every week, fewer than 15 primetime scripted TV shows have ever reached 300 episodes. And on March 15, 2016, the long-running CBS Navy crime drama "NCIS" joins them.

Since its inception in 2003, "NCIS" has maintained both a huge American audience and four of its original cast members (Mark Harmon, Michael Weatherly, Pauley Perrette and David McCallum). Sean Murray joined the cast partway though Season One.

This 300-episode milestone is even more impressive when one considers the fact that "NCIS" is a one-hour program, and that it is still - in its 13th season - the number one drama on TV.

12. "ER"

Before anyone had ever heard of McDreamy... or McSteamy... or Seattle Grace... the biggest medical drama on TV was "ER." Starring George Clooney, Julianna Marguiles, Maura Tierney and Noah Wyle, the series ran from 1994 to 2009, racking up 331 episodes.

11. "CSI: Crime Scene Investigation"

"CSI: Crime Scene Investigation," a police procedural about a team of forensic experts working cases in Las Vegas, racked up 335 episodes during its time on CBS. Starring Ted Danson and George Eads, the series ran from 2000 to 2015.

10. "Knots Landing"

"Knots Landing," a popular soap opera spinoff of "Dallas," ran on CBS primetime from 1979 to 1993, racking up 344 episodes.

9. "Dallas"

"Dallas," a primetime soap opera about wealthy oil tycoons and warring Texas families, aired on CBS from 1978 to 1991. During that time, it chalked up 357 episodes, and one very tantalizing question: Who shot J.R.?

8. "My Three Sons"

"My Three Sons," the lighthearted story of a widower (Fred MacMurray) raising his three boys, aired on ABC from 1960 to 1965, and then on CBS from 1965 to 1972. During that time, the situation comedy racked up 380 episodes (more than 125 for each son!). And when eldest son Mike (Tim Considine) married and left the series, another boy was adopted, keeping the show's title honest.

7. "Law & Order: SVU"

The first spinoff of the hugely successful crime drama "Law & Order," "Law & Order: Special Victims Unit" has aired on NBC since 1999. Over the last 17 years, it has racked up 383 episodes. It has also maintained original cast member Mariska Hargitay as Detective Olivia Benson for all 17 seasons.

6. "Bonanza"

"Bonanza," the second-longest-running western series of all time, is a period drama about a wealthy ranch family living near Lake Tahoe in the 1860s. It aired on NBC from 1959 to 1973, clocking in at 431 episodes.

5. "The Adventures of Ozzie and Harriet"

"The Adventures of Ozzie and Harriet," a hugely successful sitcom adaptation of a popular 1940s radio show, aired on ABC from 1952 to 1966. And in a certain sense, it was the country's first reality TV show, in that it starred four members of the real-life Nelson family (Ozzie, Harriet, David and Rickey) as themselves.

In its 14 seasons, "The Adventure of Ozzie and Harriet" aired 435 episodes.

4. "Law & Order"

The legal drama "Law & Order" aired on NBC from 1990 to 2010. With 20 seasons and 456 episodes, it is both the longest-running crime series ever on television, and the longest running full-hour primetime series of any kind. The success of its "ripped from the headlines" concept led to a series of spinoff shows, and rendered "Law & Order" a formidable franchise.

3. "Lassie"

"Lassie," the beloved series about a loyal collie who repeatedly saves the day and the people that she loves from danger, aired on CBS from 1954 to 1971. In addition to inspiring everything from toys and comic books to novels and feature films, the hero canine racked up 547 episodes (591 if you count the final two seasons which aired in syndication). Now that's something to bark about!

2. "The Simpsons"

"The Simpsons," an animated sitcom spoofing the everyday lives of a dysfunctional middle-class family, has aired on FOX since 1989 and clocked a whopping 589 episodes. With 27 seasons and counting, "The Simpsons" has had more seasons than any other scripted primetime program in history... by a long shot. The closest two behind it only had 20.

1. "Gunsmoke"

Inspired by the classic radio drama, "Gunsmoke," the highly-acclaimed TV series depicting the settlement of the American West, aired on CBS from 1955 to 1975. During those 20 seasons, it racked up an astounding 635 episodes, the most of any scripted primetime series ever.

So, those are the 13 scripted TV shows that have surpassed the big 300-episode milestone. But a few other television favorites are not far behind. Want to see what's next? Click on...

"Family Guy"

"Family Guy," the popular adult animated sitcom created by Seth McFarlane, has been on the airwaves since 1999. With 14 seasons and 262 episodes already on the books, Stewie the diabolical toddler and Brian the talking dog are on schedule to officially make their way onto this list in just under two years.

"Grey's Anatomy"

The popular Shonda Rhimes medical drama "Grey's Anatomy" has been on the air since 2005. As such, it already has an impressive 12 seasons and 257 episodes under its belt. So, barring any future ratings drops or cancellations, the show made famous by Meredith and McDreamy's romance is on course to hit TV's big 300-episode milestone in about 2 years.

"Criminal Minds"

As of March 2016, "Criminal Minds," CBS' long-running crime series about the FBI's Behavioral Analysis Unit, is going strong at 11 seasons and 249 episodes. With around 25 episodes in a season these days, the popular police procedural drama will likely hit the pivotal 300-episode mark in about two years.