<input type="button" value="Back to Previous Page" onclick="javascript: history.go(-1)">

{
      Button1.Attributes.Add("onClick", "javascript:history.back(); return false;")
}

static string prevPage = string.Empty;

protected void Page_Load(object sender, EventArgs e)
{
     if( !IsPostBack )
     {
         prevPage = Request.UrlReferrer.ToString();
     }
}

protected void Button2_Click(object sender, EventArgs e)
{
      Response.Redirect(prevPage);
}

For a server-side "Back"-function use:
Response.Redirect(Request.Referrer.AbsoluteUri);
This only works if it isn't a postback. If your page does postbacks use:
if(!IsPostBack)
{
ViewState.Add("ref",Request.UrlReferrer.AbsoluteUr i);
}
protected void Page_Load(object sender, EventArgs e)
{
     if( !IsPostBack )
     {
         prevPage = Request.UrlReferrer.ToString();
     }

}
Response.Redirect((string)ViewState["ref"]);