Criminal acts of sexual violence generally fall into three categories: exposure, contact, and penetration. While prosecutors introduce evidence to establish the statutory elements at trial, defense strategies focus on targeting any vulnerability in that evidence. Where the charged offense includes an element of penetration, defenses may also include specific challenges to the prosecution’s ability to prove that penetration occurred. If the prosecution is unable to prove the element of penetration beyond a reasonable doubt, the accused will be acquitted or convicted of a less serious offense. This Strategies in Brief explains the legal requirements for establishing penetration in sexual assault prosecutions and offers strategies for effectively identifying, evaluating, and presenting evidence of penetration. The article: summarizes the categories of criminal sex offense statutes and outlines the legal requirements to establish penetration; provides strategies to prepare for and try sexual assault cases involving penetration; identifies and offers guidance for responding to common defense challenges to establishing penetration in sexual assault cases.