North Korean officials say these people are criminals who have received due process under Pyongyang's Soviet-style judicial system.
American prisoners have previously been released after high-profile visits or communications from the US. Even if the individuals are merely pawns in a wider political game, the impact on their lives is great.
North Korea said last year it would treat US detainees under its "wartime law," after Washington added leader Kim Jong Un's name to the sanctions list for alleged human rights abuses and censorship.
At the time, state media KCNA reported "The Republic will handle all matters arising between us and the United States from now on under our wartime laws, and the matters of Americans detained are no exception to this."
The change was unwelcome news for US citizens already in custody, and served as a warning to others who might follow.
The latest travel warning from the US State Department
lists acts that have been treated as crimes in North Korea, whether or not they were done knowingly. They include taking unauthorized photographs, shopping at stores not designated for foreigners and carrying out religious activities.
Of the four Americans currently being held, two were said to have committed "hostile acts" against North Korea. University of Virginia student Otto Warmbier removed a political banner from a hotel
; Kim Sang Duk, also known as Tony Kim, was accused of attempting to overthrow the government. Kim Dong Chul was sentenced for being a spy. There is no information yet about possible allegations against Kim Hak-sung, who was detained this month.
The visitors are often seized at Pyongyang airport as they prepare to board flights to leave North Korea. A British journalist for the BBC recently missed his flight out when he was held and questioned
for eight hours at the end of an authorized reporting trip.
At other times, seizures are made on the border. In 2009, journalists Laura Ling and Euna Lee were arrested on the border with China and accused of entering North Korea illegally.
Where are they taken?
Initial interrogations can happen in conference rooms of hotels. While they can go on for hours or days or longer, international prisoners are generally kept in hotels at least until they are charged.
CNN's Tim Schwarz and Will Ripley, who have visited North Korea on many occasions and have even interviewed American detainees before and after trial, say international detainees can have access to local newspapers and television
during that time.
What kind of a trial do they get?
By the time a trial is scheduled, guilt is often admitted, which makes the only unknown the sentence that will be handed out. But previous prisoners have said their confessions were not made voluntarily
. And when CNN has interviewed detainees, North Korean officials have always been in the room, making it impossible to judge if statements of guilt and regret are being made under duress.
With Otto Warmbier, for example, he admitted in an emotional press conference that he had tried to steal a political banner and blamed US officials. "I never, never should have allowed myself to be lured by the United States administration to commit a crime in this country," he said tearfully as he begged for forgiveness.
Later, his trial was said to have lasted only about an hour
. In the court, North Korean officials presented fingerprints, photos of a political banner and surveillance images -- proof, they said, that Warmbier committed crimes against the regime.
Where do they serve their sentences?
Warmbier was sentenced to 15 years' hard labor -- which can mean grueling farm work.
Kenneth Bae, who was arrested in November 2012 and later sentenced to 15 years' hard labor for committing unspecified "hostile acts" in the country, said he worked outside. "I worked from 8 a.m. to 6 p.m. at night, working on the field, carrying rock, shoveling coal," Bae said on CNN's "New Day" after his release
Adding to the physical pain was the verbal abuse he received from North Korean officials, Bae said.
He said one prosecutor repeatedly told him, "'No one remembers you. You have been forgotten by people, your government. You're not going home anytime soon. You'll be here for 15 years. You'll be 60 before you go home.'"
During his captivity, Bae was presented for a CNN interview, when he told Ripley he often needed hospital treatment
"I've been going back and forth between hospital and to the labor camp for the last year and a half," Bae said in 2014.
The then 46-year-old, who suffered from diabetes, high blood pressure and kidney stones, said his health was failing.
"My hands are numb and tingling, and it's difficult sleeping at night, and I was working in the field every day," Bae said. By the time he was released, Bae had lost 60 pounds in weight.
Journalist Laura Ling, arrested with colleague Euna Lee for entering the country illegally, was sentenced to hard labor but never delivered to a prison camp. Still, she told her family of the deprivations, remarking at the time of her release that she was looking forward to eating fresh fruit again, as so many meals over the previous months had been rice, often containing rocks
Canadian pastor Rev. Hyeon Soo Lim, who is still detained, said he dug holes in an orchard for eight hours a day.
As a Canadian, he has had occasional consular visits and some letters from family. American detainees may get visits from Swedish officials on behalf of the US, which does not have diplomatic relations with North Korea.
How do they get out?
In keeping with the view that Western detainees, particularly Americans, are held as part of a bigger political scenario, it is often some grand gesture that precedes a release.
For Ling and Lee, it took a visit by former US President Bill Clinton to then North Korean leader Kim Jong Il to get them freed.
Bae was released in November 2014 after Director of National Intelligence James Clapper
went to Pyongyang as an envoy of President Barack Obama. He was released alongside another American detainee, Matthew Todd Miller.
What about North Koreans?
Tens or even hundreds of thousands of North Korean political prisoners are thought to be condemned to living in massive prison camps, according to Amnesty International
There, torture, rape, starvation and death cannot be escaped, the advocacy group says, and, unlike for American detainees, there is little or no hope of reprieve.
In 2013, Amnesty reviewed satellite images and said they identified that two of the largest prison camps had new housing blocks, expanded production facilities and enhanced security
Pyongyang officially denies the prison camps exist.