A statute of limitations puts a cap on the amount of time that can pass between allegedly illegal or criminal events and the moment that one files a lawsuit regarding those events. Most lawsuits are pursuant to state law; therefore statutes of limitations vary from state to state. Many kinds of crimes and civil infractions are subject to an expiration date on filing your complaint, including sexual assault, loan defaults, malpractice, negligence, or breach of contract. We have statutes of limitations because the more time that elapses, the harder it is for the accused to defend themselves with evidence and witnesses.
If you are seeking to bring a civil suit against someone, you should be aware of any statute of limitations that might affect the claim so that it doesn't get dismissed from court. Since the laws are subject to interpretation, the best advice you can get is from a lawyer familiar with your particular jurisdiction, as well as the circumstances of your case. The calculation of the exact time period may vary depending on these factors. Of course, if someone has brought a claim against you, you should also be sure it is valid and that the time period hasn't expired.
A statute of limitations might have some flexibility when, for example, a victim is under sixteen at the time of the crime, or if the very nature of the crime occurred over a longer period. In sexual assault, the 15-year statute of limitations in Massachusetts either begins after the rape or after the victim's 16th birthday. Especially with medical malpractice suits, an initial injury or faulty treatment may take some time to recognize. In this case, the accuser has as long as it would take a reasonable person to discover and connect their injury to a health care worker's negligence.
Some lawsuits or crimes do not have a statute of limitations, either because they are so serious that justice can be served even many years afterwards, or because there will probably be minimal evidence brought to bear. For instance, murder can be prosecuted on an unlimited time scale. A no-fault divorce can be filed at any point past the date of marriage. There is also no limit for the government collecting federal income taxes or financial agencies getting repaid student loans.
A statute of limitations protects everyone from the extended threat of a lawsuit being filed against them when circumstances prevent them from adequately defending themselves. They may have thrown out receipts, once-clear memories have fogged, or people who could have testified on their behalf are no longer in touch. This system also stabilizes and universalizes the whole process of prosecution.