If the judge has discretion to determine the sentence, the defense may bring to a judge's attention an infinite number of factual circumstances that may move the judge to impose a lighter sentence. The following are examples of such circumstances (called "mitigating" factors):

• the offender has little or no history of criminal conduct
• the offender was an accessory (helped the main offender) to the crime but was not the main actor
• the offender committed the crime when under great personal stress; for example, had lost a job, was late on rent, and had just been in a car wreck, or
• no one was hurt, and the crime was committed in a manner that was unlikely to have hurt anyone.

  Just as mitigating circumstances can sway a judge to lessen a sentence, "aggravating" circumstances can compel a judge to "throw the book at" an offender. A previous record of the same type of offense is the most common aggravating factor. Other aggravating circumstances grow out of the way a crime was committed, as when an offender is particularly cruel to a victim. Sometimes, laws themselves specify aggravating factors, such as the use of a weapon.

As criminal courts become ever more crowded, prosecutors and judges feel increased pressure to move cases quickly through the system. Trials can take days, weeks or sometimes months while guilty pleas can often be arranged in minutes. Also, the outcome of any given trial is usually unpredictable -- but a plea bargain provides both prosecution and defense with some control over the result.

In theory, a plea bargain may be negotiated at any time after arrest. In practice, however, the time to plead depends on the court and the jurisdiction -- some jurisdictions allow plea bargains only during certain phases of the criminal process. In many other places, however, plea bargains can be worked out virtually any time -- from shortly after the defendant is arrested (before the prosecutor files criminal charges) up to the time a verdict is reached -- even during trial itself. Also, if the trial results in a hung jury (the jurors are split and cannot make the unanimous decision required), the prosecution and defense may (and often do) negotiate a plea rather than go through yet another trial.