Notification and response system to enable applications to communicate with recipients using different media, by: sending requests to one or more recipients, using the medium specified by each individual recipient; collecting and processing responses; and forwarding responses to their final destination using the medium specified by the final destination. The request is delivered to a recipient according to their preferences. Communication flow expressions specify the recipients, and how, when and where each recipient receives the request. Requests are dynamically updated, and parameters of a communication flow expression are not evaluated, until the request is delivered. Communication flow rules specify recipient's communication preferences and tailor communication flows to characteristics of the sender, the topic or scheduling constraints. Communication flow expressions are evaluated using a three-valued logic of notification success, failure, and possible failure. Primitives specify simultaneous or sequential contact, and when execution of the sub-expression should terminate.