In the tricky business of ranking the most-watched TV series in the world, two CBS shows seem to perennially appear near, or at the top, of everyone’s estimations: “NCIS” and “CSI.”

“No one knew [“CSI”] was going to be the juggernaut it became — we just all knew it was unique and beautifully produced and peeled back the curtain on a world and job never seen before,” says CBS Television Studios president David Stapf. He joined the company in 1999, the year before “CSI” premiered, and helped shepherd the show in its infancy. “We knew it was good, but a lot of things are good and don’t resonate with viewers, so it was gratifying when those early ratings came in.”

“NCIS” was a slower build for CBS. The first signs that the show would become a ratings juggernaut came not on CBS, but from overseas networks that carried the show. Armando Nuñez, CEO of CBS Global Distribution Group, recalls seeing “NCIS” deliver impressive ratings in France. And then the show began to pop in Germany.

“I remember having a conversation with [Stapf] saying, ‘Hey you should pay attention to what this show is doing in Europe,’ ” Nuñez recalls. “And then we saw the ratings starting to grow in the U.S.”

“NCIS” had been a tough sell in the U.K. at the time because there was an aversion in the culture to U.S. military themes. But Nuñez persuaded a buyer at Channel 5 to give it a shot by making him an offer he couldn’t refuse.

“We didn’t give it away, but we did a deal at a lower license fee just to get it on the air,” he says. “And it’s been one of Channel 5’s most popular shows for many years.”

Stapf says he sees several clear reasons behind their global success, one of which is the sales efforts by Nuñez and his group.

“I think Armando and his team do a phenomenal job working with their clients in creating an environment where the shows are set up for success in their individual markets,” Stapf says. “Their relationships with the buyers are key to all of it.”

Stapf also credits the series’ “unusual” subject matter and “compelling” characters as playing a large part.

“The job of forensic scientists was unusual, and I think audiences appreciate that there are people consumed with finding the truth through the evidence. Also, the characters were interesting and compelling and always came out on top. I think that resonates with all audiences, not just a U.S. one.”

The connection with audiences around the globe has translated into “NCIS” being widely estimated as the most-viewed TV series in the world over the past few years. The show is licensed in more than 200 markets worldwide and has been dubbed in over 60 different languages.

With both shows, CBS found a formula for success that it has used to spawn a bevy of spinoff series (“Miami,” “New York” and “Cyber” for “CSI”; “Los Angeles” and “New Orleans” for “NCIS”). However, Stapf says the network has always been keen to make sure that each new series “provides something new and unique to the mothership.”

“In the case of ‘CSI,’ both ‘Miami’ and ‘New York,’ and then ‘Cyber,’ were their own shows that had the original spine of forensic science, but also had completely different tones, looks and styles,” Stapf explains. “The saturated neon-like colors of Miami and the brevity of language from David Caruso, with ‘New York,’ the grit and underbelly of the city played a huge role, the intricacies of crime in a city were so different from the crime in Las Vegas.”

Given that the international popularity of either show is likely to wane anytime soon, CBS is on the hunt for its next global viewership phenomenon. According to Stapf, several of the network’s other shows, including “Hawaii Five-0” and “Elementary,” have been “extremely successful in the international marketplace, while they are also “seeing signs” freshman drama “FBI” from executive producer Dick Wolf is “following a similar trend of interest” globally.