Bounties

A Bounty is a set amount of money placed on a persons head for their death/capture.

Becoming Wanted
Normally a bounty is issued by a government official or by the police. The idea is to tempt others to aid with the capture of the said criminal so he can be brought to justice quickly. The money often leads people, known as bounty hunters to make careers out of trying to find these wanted people. For the Marines, this is their aid in bringing in criminals and enforcing their definition of "Justice". In the Whiskey Peak arc, the Baroque Works agents state that up to 30% of the bounty money may be lost by bringing in someone dead as opposed to bringing them in alive, as the government prefer to hang them in public.

In the eye of a pirate, a high bounty may actually be a source of pride for them, as seen in Luffy's enthusiasm on his first bounty, or Sanji's constant complaining of his lack of bounty. For others, such as a Kuro, it be can an annoyance.

Bounty Issuing
The average bounty in the world is 10-20 million Beli, as stated by one of the officers when announcing Luffy's bounty. It is important to remember that a bounty does not always reflect the strength of a person, as was confirmed by Oda himself in his SBS section. Simply doing criminal activities, associating yourself with certain groups or participating in forbidden practices (such as researching the Poneglyphs) can make you gain or increase a bounty without reflecting the strength of the person carrying it. Even performing outrageous acts dedicated to self-preservation can cause people to be assigned with bounties, such as when the Straw Hats survived the 2nd buster Call in known history.

In short, the bounty simply reflects how much threat (or potential threat) the said person has to the World Government. However, the bounty is based only on the known information about the criminal. In the case of Luffy both Kuro and Enel did not contribute to his bounty as Kuro was believed to already be dead and Skypiea remains relatively unknown to the majority of the One Piece world. Another example would be Arlong, who paid the corrupt Marine Nezumi money to ignore his actions in the East Blue, thus his bounty did not reflect his potential threat.

Retracting Bounties
A bounty is usually given to a criminal for life, so no matter which lifestyle they may follow after its issue, the bounty remains unchanged. The bounty is usually only written off when the criminal is either captured or dies. The notion that death brings freedom from the bounty was the sole reason for Kuro's plan to escape a pirates life.

With exceptions of death and capture, another method of losing a bounty is to become an asset to either the Marines or World Government. Every member of the Shichibukai is considered an ally of the Marines and World Government so their bounties were all removed. This allows them to continue being a criminal on a conditional basis.

A bounty may also be removed by a court of law, as Jango's was, during his mini-series after he hypnotised the entire courtroom to dance. Jango's case was further aided by the help and plea of the Marine Fullbody during the case. This later allowed him to later join the Marines under the command of Hina.

In the Little Garden arc, it is revealed that a bounty can last for over a hundred years if the criminal is still alive and not captured.

Identification
A criminal is, most often than not, known by his poster. Without the issuing of a wanted poster, no criminal can be made known. These posters are distributed across the world in newspapers and bars. Marines also keep copies of each criminal at their bases and on board their ships so the Marines always have them handy. Bounty Hunters may also carry a list of posters showing the criminals they're searching for and ones they've already killed.

The Poster Picture
The most important part of a poster is the picture of the criminal. It allows swift identification upon sighting and informs everyone in the world to be wary of the named criminal. The picture normally takes the form of a photograph, in failing to retrieve a suitable photograph, a drawing may be used instead as in the case of Sanji. To some (such as Sanji), a decent photograph is as important as the bounty itself as the entire world will see copies of it.

One of the most mysterious things about wanted posters in One Piece is how the Marines are able to get hold of photos of almost every criminal. Oda later in one of his SBS later reveals that the photos are taken by the Marine Photography Branch Captain "Flaming Attachan". Flaming Attachan apparently gets his name because before he closes the shutter he yells "Fire!".

The only exception to this rule seems to be from Mr. 13 and Miss Friday who were used by the marines recently to help draw bounty posters of remaining Baroque Works agents, a new picture is sometimes issued as the persons appearance may have changed (such as Robin).

Nicknames
Marines give pirates a nickname with a bounty that either describes their appearance (such as "Straw Hat Luffy" or "Cyborg Franky"), what their powers/abilities are (such as "Black Leg Sanji" or "King of Snipers Sogeking"), or what they do (such as "Burglar Cat Nami" or "Pirate Hunter Zoro"). Sometimes it is just the most common nickname a person has (Luffy was called "Straw Hat" before his first bounty was issued).

The Straw Hat Pirate crew
Total bounty: 700,000,050 Belli

East Blue Bounties
Note: The average bounty for the East Blue is rated at only 3,000,000 Belli, making it the weakest sea in the One Piece world.

Shichibukai ex-bounties
Because they are frozen bounties, the Shichibukais bounties do not reflect their true threat anymore.

Trivia

 * Bounties are often the centre of several myths within the fandom.