Originally Posted by Fatbrit

I see by looking through your other posts that you are British. You'll need to order a "police certificate" from: Association of Chief Police Officers

Do note that this is a new system. Previously, you could order a certificate from your local police -- but the US Consulate in London no longer accepts these.

If you've lived in ay other countries for more than 6 months, you'll also probably need a certificate from them, too.