Someone has a sex video of me and will not return it to my possession, what can i do legally?

You might want to also post this question in another section of this Q&A. Maybe in one of the civil law sections. I'm not sure this is a true criminal law question... If you made the video knowingly and voluntarily (and you're of legal age to consent to the activities depicted on the video)...I don't think it's a crime to merely possess the video. If you're saying he/she stole the video from you, you could accuse this other person of stealing the video (which of course is a crime). However, if the person who has the video is also IN THE VIDEO...he/she could make an argument that it's just as much his/hers than it is yours. A video depicting a performance of two consensual adult participants is not realistically viewed the same way as say a car, a cell phone, an Xbox. We know who owns the car, the cell phone, the Xbox, right? This kind of video is similar to a wedding portrait. Who owns it?...the bride or the groom...or do they BOTH have a claim for it? Who knows? For this reason, if you called the police about this kind of theft, they probably would tell you to file a lawsuit in small claims court. They are not going to want to get involved in this sort of matter unless you give them reason to believe the video was made without your knowledge or consent. If you're saying he/she is threatening to publish the video online or to others, again this might be more of a civil law question. As far as any blackmail is concerned, what do you mean?..."they are threatening to use it in court". I'm not sure a video of you engaging in sex would ever be relevant in any court proceeding. For instance, if someone were arguing that you are an unfit parent, how would showing a judge that you engage in sex be proof that you are an unfit parent? Bottom line, if you are "in court", you most likely have an attorney...speak with your attorney about this matter. If you don't have one, you may want to consider consulting with one. Good luck.