Yes, PHP SDK, sorry forgot to mention that.  Your response helped solve this issue.  There was one additional bit I needed to add in the AutoLoader include first:   use Ctct\Components\Contacts\Address;   Then, the final result for my successful code was:   $contact = new Contact();             $contact->addEmail($_POST['Email']);             $contact->addList("999999999"); // I was hardcoding the List ID in mine so I replaced with 9s for demo.             $contact->first_name = $_POST['Name'];             $contact->company_name = $_POST['company_name'];             $contact->job_title = $_POST['Title'];             $contact->work_phone = $_POST['Phone'];             $state = $_POST['State'];             $contact->addAddress(Address::create( array("address_type"=>"BUSINESS","state"=>$state)));   THANK YOU! ... View more