Problem with Outlook

Hi,
I get this error
Error    2    The type or namespace name 'Office' does not exist in the namespace 'Microsoft' (are you missing an assembly reference?)    C:\App17\App17\Default.aspx.cs    22    27   App17

due to this line

using Outlook = Microsoft.Office.Interop.Outlook;

within VS 2013 project. why?

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Many Thanks & Best Regards, Hua Min