'Session' does not exist in the current context

I have the following code, that uses session but i have an error in the line :

if (Session["ShoppingCart"] == null)

the error is cS0103: The name 'Session' does not exist in the current context what is the problem ?

using System;
using System.Data;
using System.Configuration;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;

using System.Collections.Generic;
using System.Web.SessionState;
/// <summary>
/// Summary description for ShoppingCart
/// </summary>
public class ShoppingCart
{
    List<CartItem> list;
    public ShoppingCart()
    {
        if (Session["ShoppingCart"] == null)
            list = new List<CartItem>();
        else
            list = (List<CartItem>)Session["ShoppingCart"];
    }
}