In which namespace can the class "Session" be found

Dear all

I need to use session variables in my program but I can not find the Session class.
I need to know in which namespace of the .Net framework the Session class can be found.

Thank you guys in advance

protected void Read_Emails(object sender, EventArgs e)
        {   
            Pop3Client pop3Client;
            if (  
                Session["Pop3Client"] == null)
            {
                pop3Client = new Pop3Client();
                pop3Client.Connect(txtMailServer.Text, int.Parse(txtPort.Text), chkSSL.Checked);
                pop3Client.Authenticate(txtUserName.Text, txtPassword.Text);
                Session["Pop3Client"] = pop3Client;
            }
            else
            {
                pop3Client = (Pop3Client)Session["Pop3Client"];
            }
            int count = pop3Client.GetMessageCount();
            DataTable dtMessages = new DataTable();
            dtMessages.Columns.Add("MessageNumber");
            dtMessages.Columns.Add("From");
            dtMessages.Columns.Add("Subject");
            dtMessages.Columns.Add("DateSent");
            int counter = 0;
            for (int i = count; i >= 1; i--)
            {
                Message message = pop3Client.GetMessage(i);
                dtMessages.Rows.Add();
                dtMessages.Rows[dtMessages.Rows.Count - 1]["MessageNumber"] = i;
                dtMessages.Rows[dtMessages.Rows.Count - 1]["Subject"] = message.Headers.Subject;
                dtMessages.Rows[dtMessages.Rows.Count - 1]["DateSent"] = message.Headers.DateSent;
                counter++;
                if (counter > 5)
                {
                    break;
                }
            }
            gvEmails.DataSource = dtMessages;
            gvEmails.DataBind();
        }

Here Session is giving error as not present in current context.

Tags: C#, ASP.NET, Session