A public service announcement (PSA) is a message in the public interest disseminated without charge, with the objective of raising awareness of, and changing public attitudes and behavior towards, a social issue. In the UK, they are generally called 'public information films' (PIFs); in Hong Kong, they are known as 'announcements in the public interest' ('APIs').
The earliest public service announcements (in the form of moving pictures) were made before and during the Second World War years in both the UK and the US.
In the UK, amateur actor Richard Massingham set up Public Relationship Films Ltd in 1938 as a specialist agency for producing short educational films for the public. In the films, he typically played a bumbling character who was slightly more stupid than average and often explained the message of the film by demonstrating the risks if it was ignored. The films covered topics such as how to cross the road, how to prevent the spread of diseases, how to swim, and how to drive without causing the road to be unsafe for other users. During the war, he was commissioned by the Ministry of Information to produce films for the war effort. Massingham began to produce longer films, for both private companies and the Government, after the War.
In the US, the Ad Council (initially called the War Advertising Council) was set up in 1941, when America entered World War II. It began implementing on a massive scale the idea of using advertising to influence American society on a range of fronts. Its first campaigns focused on the country's needs during World War II, such as encouraging the American public to invest their savings in government bonds.
After the war, PSAs were used to educate the public on a broader range of important issues. In the UK, they were produced for the Central Office of Information (COI), and again by private contractors, which were usually small film companies, such as Richard Taylor Cartoons. They were supplied to broadcasters free of charge for them to use whenever they wished. Their usefulness as a cost-free means to fill the gaps in fixed-duration commercial breaks left by unsold advertising airtime led to their being used regularly and extensively in the 60s, 70s, and much of the 80s, and consequently, within both the COI and broadcasting companies, they were typically known as "fillers". They are still being produced, although the vastly reduced need for broadcasters to turn to third-party filler material to deal with unused airtime during breaks or junctions means they are now only seen rarely.
The most common topics of PSAs are health and safety, such as the multimedia Emergency Preparedness & Safety Tips On Air and Online (talk radio/blog) campaign. A typical PSA is part of a public awareness campaign to inform or educate the public about an issue such as obesity or compulsive gambling. The range of possible topics has expanded over time.
One of the earliest television public service announcements came in the form of Smokey Bear whose "Only you can prevent forest fires" campaign became famous and ran for decades.  Other common early themes were traffic safety, especially safe crossing, wearing seat belts and not driving drunk, and the dangers of cigarette smoking.
From time to time a charitable organization enlists the support of a celebrity for a PSA; examples include actress Kathryn Erbe telling people to be green and Crips gang leader Stanley Williams speaking from prison to urge youths not to join gangs or commit crimes. Some PSAs tell people to adopt animals instead of buying them. Protecting our Earth, also known as being green, is another example of a current PSA topic.
Some television shows featuring very special episodes made PSAs after the episodes. For example, Law & Order: Special Victims Unit talked about child abduction in one episode, so it had a PSA about child abduction. Another example is when the original Law & Order did an episode about drunk driving, which had a PSA about drunk driving.
During the 1980s, many American cartoon shows contained PSAs at the end of their shows. These may or may not have been relevant to the episode itself. Three of the most widely known are the closing moral segments at the end of He-Man and the Masters of the Universe, the "Knowing is Half the Battle" epilogues in G.I. Joe: A Real American Hero, Jem, M.A.S.K., Inspector Gadget, and the "Sonic Says" segments from Adventures of Sonic the Hedgehog.
Some television PSAs have topics such as on not watching so much television, or not taking fictional stories literally; or about television, movie, or video game ratings. During the late 1980s and early 1990s, environmental-political issues became popular, such as the Turtle Tips or Planeteer Alert. The English dub of Sailor Moon also has the PSA closing segments at the end of each episode called "Sailor Says" (known as "Sailor Moon Says") in two seasons only.
China's first PSAs in 1986 were about saving water and were broadcast on Guiyang television. In Hong Kong, terrestrial television networks have been required since National Day 2004 to preface their main evening news broadcasts with a minute-long announcement in the public interest which plays the Chinese National Anthem in Mandarin over various patriotic montages.