The estoppel certificate is usually a letter that you get from all of the tenants in commercial real property when you’re under contract to purchase it. It’s important, because in addition to looking at the leases, you really want to know what the tenant thinks his deal is with the landlord. So, for example, the leases will tell you the terms and the rent, but the estoppel certificate will tell you if the tenant has any claims against the landlord.