There's an event on the dropdownlist called SelectedIndexChanged
] which fires up when a value has been selected on the control. That event for your Countries control should be existing on your code behind as that is where you will put the logic for retrieving the cities from your database. I assume you have a foreign key of the country on your Cities table so the retrieval is pretty straightforward. Also, you need to set Autopostback to true on the HTML/ASP.Net tag of your control so that the event will be fired.