The type or namespace name 'Practices' does not exist in the namespace 'Microsoft'

I am using Microsoft Visual Studio 2005 for c# I have the following namespace on my code

using Microsoft.Practices.EnterpriseLibrary;

using Microsoft.Practices.EnterpriseLibrary.AppSettings; 

using Microsoft.Practices.EnterpriseLibrary.Data;

and I keep on getting this error:

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

What should I do?

Thanks in advance. :)