Dear all, I am a new user of idl. I have this task to perform: - create a button which will call and execute an exe file. - I already created the button but i have difficulties to associate an event to this button. My source code is the following, should somebody help to find what is wrong? thanks a lot.  Josiane.     ; /CDECL, /VERBOSE, /SHOW_ALL_OUTPUT) EQ 1) (CALL_EXTERNAL('C:\Josiane\ITK\Projects\Reg_Test\debug\RegHello.exe', 'Registration', C:\Josiane\ITK\src\InsightToolkit-3.8.0\Examples\Data\BrainProtonDensitySliceBorder20.png, C:\Josiane\ITK\src\InsightToolkit-3.8.0\Examples\Data\BrainProtonDensitySliceShifted13x17y.png, C:\Josiane\ITK\Projects\Reg_Test\Results\Res.png,$