The Cascading DropDownList is the series of DropDownList, where the value of one DropDownList depends on the another DropDownList value. This can be configured by using the change event of parent DropDownList. Within that change event handler, you should load the data to child DropDownList based on the selected value of parent DropDownList. In this sample, if a country is selected from countries DropDownList, the respective states will be loaded in the second DropDownList and the same has to be done between states and cities DropDownList. More information on the Cascading feature configuration can be found in the documentation section.