The type or namespace name 'Practice' does not exist in the namespace 'Microsoft.Practices.EnterpriseLibrary...' (are you missing an assembly reference?)

 Microsoft Enterprise Library

Error: Are you getting "The type or namespace name 'Practice' does not exist
in the namespace 'Microsoft.Practices.EnterpriseLibrary...' 
(are you missing an assembly reference?)" 
even if you have add the .dll  to your project?

Reading documentation 

Microsoft Enterprise Library 6.0 is not backwards compatible and does not support multi-release .NET Frameworks.  It only supports .NET Framework 4.5.

Microsoft Enterprise Library 5.0 supports .Net 4.0 & .Net 3.5 SP1

Solution Fix: 

Another common fix for the "The type or namespace name 'Practice' does not exist in the namespace 'Microsoft.Practices.EnterpriseLibrary...' (are you missing an assembly reference?)" is always compile target framework with full 4.0 .NET Framework and NOT .NET Framework 4 Client Profile, the savings in build size are minimal.