Drillthrough reports : These are the reports which gets opened up when user clicks the link on the main report. Its usually shows the details of the summary on the main report. [ReportViewer.Drillthrough] Event is used to handle the report data population on detail drill through report. Drill through report can be the same main report with different parameters \ data source or a completely new report.

void MyDrillthroughEventHandler(object sender, DrillthroughEventArgs e)        

 {             // Modify the data source of same main report or load a new report         }                  // Add the handler for drillthrough

event.ReportViewer1.Drillthrough += new DrillthroughEventHandler(MyDrillthroughEventHandler);

Drilldown reports : These report layout shows the general view until user clicks on the link to view the specific record details. Data is displayed in different levels. It can be achieved in the design by adding groups and setting the row visibility properties accordingly. It is similar to group headers and details in reports.