What Is "Investment Grade" Triple net (NNN)?

Investment grade real estate refers to the tenant's credit rating.  Anything above the credit rating BBB- is considered "investment grade."  That said there are a few private companies that don't have credit ratings (because their books are not public record) that are also considered "investment grade" based on their reputation.

Below is a chart of credit ratings from Moody's verses Standard & Poors which use different terminology (Fitch and Dun & Bradstreet use the same as S&P)...