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title: "_DEBUG | Microsoft Docs"
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---
# _DEBUG
The compiler defines `_DEBUG` when you specify the /MTd or /MDd option. These options specify debug versions of the C run-time library.  
  
 For more information, see [CRT Debugging Techniques](/visualstudio/debugger/crt-debugging-techniques).  
  
## See Also  
 [Control Flags](../c-runtime-library/control-flags.md)