How to get current logged on user id?

I am using the built in membership tools.. the userID is stored in a database table called aspnet_Users..

so what goes in here?

protected void button1(object sender, EventArgs e)
    {
        string comment;
        int rating;
        string gameTitle;
        string userb;

        comment = commentBox.Text;
        rating = ratingList.SelectedIndex;
        gameTitle = Request.QueryString["filename"];
        userb = ????

Where I have userb should be to retrieve the current user,

thanks