I have a case where the defendant and victim have emailed each other, or are communicating through instant messenger. The victim has provided me with a transcript of the messages. Assuming they are relevant, how do I get them into evidence? Is it like any other letter, and how does the court determine they are authentic?

Take a look at the new Predicate Manual. It has some similar types of evidence and the most recent cases.

Can you ask about them, have him deny them, then have your witness speak of them as an inconsistent statement? You would really have to have a computer expert admit them otherwise.

And, if he does not take the witness stand, your witness could testify that a conversation took place on certain date(s), and as to your witnesses statements, and understanding of the conversation (without the heresay objection). Let the jury fill in the blanks. When done right, what isn't said can look worse than what is said.

Getting the conversation (which is all it really is) into evidence is no different than one had over the phone. But, the tricky part is establishing that the defendant was party to one end of the conversation. Normally, that is accomplished by having the other party identify the voice.

Surely, you can establish his messaging name is connected to him or his computer. In addition, there may have been identifying remarks in the conversation, including references to things only the defendant would know or say.

A good recent case on the subject is Massimo v. State, No. 2-03-318-CR (Tex. App. -- Ft. Worth 8/5/04).