some times we also use Caching Technique to manage the State in outproc medium like M-Cache N-Cache but its use in Large enterprise applications

InProc, state mantain in the memory of the ASP.NET process 

And OutProc, session  managed by an external resource (

Like StateServer or SQL Server 

and Inproc session states is faster than outproc 

Inproc not need to be serialized