Similar Acts Evidence, or Rule 404(b) Evidence, is a type of evidence used in the United States Federal Rules of Evidence. It is evidence of a person's prior bad acts, crimes, wrongs, or other misconduct that is used to prove that a person acted in a certain way or had a certain intent on a particular occasion. It is typically used to prove that a person acted with a particular state of mind, such as intent or knowledge, or that a person had a propensity to commit a certain type of crime. There are two types of Rule 404(b) Evidence: propensity evidence and modus operandi evidence. Propensity evidence is evidence of past acts that are offered to prove that the defendant has a propensity to act in a certain way, such as a propensity to commit crimes. Modus operandi evidence is evidence of past acts that are offered to prove that the defendant committed the crime in question in a particular manner, such as a certain type of modus operandi.