A portable electronic device with a touch screen display displays an incoming message from a party to a user of the device while the device is locked. The device responds to detecting a user request to view the incoming message by immediately displaying a set of messages between the user and the party in a chronological order. While the device is unlocked, if an incoming message is longer than a predefined length the device displays a portion of the incoming message, a view icon, and a long-message indicia. Alternately, if the incoming message is equal to or less than the predefined length, the device displays the entire incoming message and a reply icon. In response to user selection of either the view icon or the reply icon, the device immediately displays a set of messages between the user of the device and the party in a chronological order.