What Is Melendez-Diaz?

The Melendez-Diaz decision by the US Supreme Court changed how forensic and scientific evidence is allowed into court. Before the decision, it was common practice for documentary evidence, such as forensic reports, DNA tests, and other scientific analysis to be used and cited directly in the case.

The court determined that this system was unconstitutional, and unfair to defendants, based on the application of the 6th amendment confrontation clause. The Confrontation Clause explicitly states that “in all criminal prosecutions, the accused shall enjoy the right…to be confronted with the witnesses against him”.

Melendez-Diaz was partially based on a previous Supreme Court confrontation clause case, Crawford v. Washington (2004). In Crawford, the court ruled that evidence that was testimonial in nature must provide a witness, but it did not explicitly say what was testimonial.

A document or a report can’t be cross-examined or challenged if it is entered into the court record as evidence as-is. Therefore, the court ruled that an expert witness must be available to explain and defend the evidence and conclusions drawn from these reports. And the defense has an opportunity to challenge the witness and point out any inconsistencies, and potentially try to suggest how the evidence could be flawed or incorrect.


The Melendez-Diaz decision has impacted how many types of criminal cases have been prosecuted. The specific cases commonly affected include:

  • drug charges, where the state must prove that a drug is actually an illegal controlled substance, and
  • drunk driving cases, where the state must establish the scientific basis of the breath test machines upon which evidence of intoxication rests.

Different district attorneys in different states have had to handle Melendez-Diaz challenges in different ways. But most everyone agrees that states now have to have more experts on hand to testify in cases where these challenges are issued.

That isn’t to say that all defense attorneys will want to cross-examine an expert. In many cases, an expert’s testimony is not going to help their client’s case, it will only further cement the strength of the scientific evidence against the defendant.

But it will certainly come up in some cases, now that it is an option. And managing the extra administrative courtwork is an issue.

Notice and Demand

To address the practical and administrative hurdles of complying with Melendez-Diaz, many states like Virginia have implemented a procedure known as notice and demand.

Notice and demand formalizes the procedure for presenting and challenging forensic evidence in court. The prosecution must provide the defense the scientific documents in advance, and the defense must notify the prosecutor and the court of their intent to challenge the documents. This way, the state has advance warning to get the required expert witnesses into court to testify and defend the evidence.

Future Changes

Despite the fact that this decision was only heard last year, the Melendez-Diaz decision is set to be reviewed and possibly changed significantly this session.

Further Reading:

A great overview of the details of Melendez-Diaz is here.