In a criminal or civil case, mitigating circumstances are the conditions, facts, events or details that may be considered as relevant and affective to such a degree that they may lessen or reduce the severity of the outcome of the case. In criminal cases the severity may be reflected in the specific classification of the offense (for example, involuntary manslaughter as opposed to first or second degree murder) as well as the number of years of incarceration imposed as punishment, (5 years compared to a life sentence) and may also affect monetary damages and the conditions of ultimate release (probation, parole, living conditions, restrictions of travel). In contrast, "aggravating circumstances" are details that usually add to the severity of the results.

Mitigating circumstances that are often considered in a criminal case may typically include factors that tend to show that the defendant does not actually pose a risk to society, or that he or she does not have a criminal history, or that he or she accepts responsibility for his or her actions in the case. Other mitigating circumstances may include the history of the circumstances between the defendant and the victim. In a voluntary manslaughter case, for example, the defendant may have been beaten, brutalized and abused by the victim from childhood until the killing. (This is not to be mistaken with or substituted for self defense, which is an affirmative defense). The mitigating circumstances may not necessarily excuse or condone the action of the defendant but they may offer some insight into the defendant's mindset. Perhaps compassion and mercy may in this way be factored into the case.

Indeed, mitigating circumstances may actually keep a defendant alive in death penalty cases. In federal cases, and in most state cases, the court will ask the defendant if he or she wishes to speak on his or her own behalf and provide any information that may mitigate the severity of the punishment. As the defendant speaks, the court may choose to take into account any overt and/or subtle nuances in tone, message, eye contact and body language that may also contribute to the reduction of the severity of the outcome. It should be noted that the victim and/or the survivors of the victim, as well as the prosecuting attorney, may also be given the opportunity to present information that may affect the final outcome of the sentencing. In some instances the relatives or friends of the victim may offer information that may actually support the defendant's efforts to mitigate the results of the case, similar to the abuse in the above mentioned example. The point here is that the defendant has the opportunity to come forth and offer such information that may mitigate, reduce or ameliorate the outcome of the case. The mitigating circumstances may be - and often are - of such a substantial nature that the verdict will result in a life sentence and not a death sentence, or in less serious cases, a reduced sentence or even an acquittal.

Finally, an attorney always has the duty to strive to mitigate or lessen the punishment or damages imposed upon his or her client. The attorney's overall goal is to reduce damages and to protect and defend his or her client. The concept of mitigating circumstances may be seen as one particular method of protecting and defending the client in both criminal and civil matters.