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More police departments are outfitting policemen with wearable cameras that tape what officers see as they do their job, providing a record in the aftermath of incidents like the one in Ferguson, Mo., where a policeman shot and killed an unarmed black teen.

The cameras have shown promise in reducing incidents involving use of force as well as citizen complaints, according to new studies. Worn on officers' lapels, glasses or hats, the cameras can document a more definitive version of police work.

"If you look at what's happening in Ferguson—basically you have two entirely different versions of events," said Michael White, a professor of criminology at Arizona State University. "If that officer was wearing a body-worn camera we could just go to the tape."

Dr. White, who has studied the use of the cameras, said "there's tremendous potential to head off these types of incidents from blowing up afterwards."

In a press conference earlier this week, Thomas Jackson, chief of the Ferguson police department, said the department had only recently been budgeted $5,000 to buy a limited number of dashboard cameras and body cameras for officers. Mr. Jackson said the department had not yet put them into use at the time of the shooting but plans on incorporating them into vehicles and on officers soon.

In the Southern California city of Rialto, the number of citizen complaints against police dropped from 24 to 3 in the first year that the patrol officers began wearing cameras in 2012. Use-of-force incidents plummeted from 61 to 25 during the same period.

Tony Farrar, Rialto's chief of police said, "When you talk about putting a camera on somebody, human nature is going to dictate that you're going mind your p's and q's and you're going to be on the best behavior."

"At the same, I think it's had an impact on citizens—if they know you're wearing a camera they too will be on their best behavior," he said.

A 2013 study found that a quarter of 254 U.S. police departments surveyed used body cameras, according to the Police Executive Research Forum. More than 1,200 law enforcement agencies have purchased wearable cameras from Taser International Inc., TASR +9.44% with about 80% of the company's camera sales occurring in the last 12 months, according to Taser spokesman Steve Tuttle.

Big city police departments have begun to use or test body cameras, including New Orleans, Los Angeles and Las Vegas.

There also has been some resistance in some quarters.

The police union in Las Vegas initially opposed using body cameras, worrying that the footage could be used in fishing expeditions against officers in cases where there were no complaints, said Chris Collins, the union's executive director. The proposal to use cameras had come after rash of police shootings three years ago. Now, some Las Vegas police have begun wearing them after the union and the department hammered out guidelines, including when supervisors are allowed to review footage, Mr. Collins said.

Police body cameras also have sparked some privacy concerns for citizens, but civil liberties groups back their use with some restrictions.

The American Civil Liberties Union said last year that the cameras have the "potential to be a win-win, helping protect the public against police misconduct, and at the same time helping protect police against false accusations of abuse."

The cameras themselves are only part of the expense—The cameras themselves are only part of the expense—Taser's cameras range from $399 to $599. Data-storage and management costs can be significant, according to a recent report by Dr. White, the Arizona State University professor. "The logistical and resource issues are especially challenging for those smaller police departments," he said.

The police department in Mesa, Ariz., did a side-by-side study of 50 officers wearing cameras and 50 without. The results after eight months: officers with cameras were subject of 8 citizen complaints while those without had 23.

With citizens regularly capturing incidents with police on their cell phones, Mesa Police Department spokesman Steve Berry said the department wanted to have their own video "to tell the whole story or certainly both sides of the story."