Many states require an “affidavit of merit” in order for a plaintiff to bring a malpractice claim (medical or legal). The blog Professional Liability Matters has prepared this 50 state survey-table with the relevant information.
One interesting question to ask is whether a claim against a professional for a different cause of action, such as a breach of fiduciary duty or an intentional tort, would require the use of a certificate of merit. Some courts have held that it would not.
UPDATE (1/27/16): In relation to the last point above, here is the most recent development. Just a few days ago, in Perez v. Zagami, LLC, 2016 BL 7198, N.J. Super. Ct. App. Div., No. A-3268-14T2, 1/12/16, the court held that the New Jersey statute that requires malpractice plaintiffs to file an
affidavit of merit attesting to the viability of the complaint doesn't
apply to a lawsuit accusing an attorney of malicious use of process. For more on the case go here.