A portable electronic device for instant messaging is disclosed. One aspect of the invention involves a graphical user interface (GUI) on a portable electronic device with a touch screen display. The GUI has a set of messages exchanged between a user of the device and another person. The set of messages are displayed in a chronological order. In response to detecting a scrolling gesture comprising a substantially vertical movement of a user contact with the touch screen display, the display of messages are scrolled in accordance with a direction of the scrolling gesture. The detecting of the scrolling gesture is substantially independent of a horizontal position of the user contact with the touch screen display.