It resides in a separate .NET framework assembly named System.Windows.Forms.DataVisualization.dll (don't look for the ".Charting", it's not there) that Windows Forms projects created by the IDE don't reference by default. So you need to either add it to your project's references or put a #using "System.Windows.Forms.DataVisualization.dll" directive (note the #) into the source file using it. I think it should also be added to the project's references automatically once you place a chart control on a form using Forms Designer, but I'm not 100% sure about that.