Character Evidence in Criminal Cases: Specific Instances of Prior Misconduct by the Accused
Other crimes or prior acts of misconduct by the defendant are not admissible during the prosecution's case in chief if the only purpose is to prove criminal disposition.
However, prior crimes or acts of misconduct may be admitted at the initiative of the prosecution when the misconduct is relevant to prove a material fact other than character or disposition, such as motive, opportunity, intent, preparation, plan, knowledge, identity or absence of mistake or accident. This also applies to civil cases.
Remember MIMIC: Motive, Intent, Mistake (absence of), Identity, Common Plan or Scheme.
The MIMIC exception is subject to FRE 403, which allows the evidence to be excluded if the probative value is substantially outweighed by the danger of unfair prejudice.