Catholic natural law has had a long and evolving interest in bioethics. Thomas Aquinas left natural law a legacy of great flexibility in evaluating goods within a whole life. He also bequeathed to the Church the basis for an abolutism on sexual issues. Modern reproductive medicine and a deeper understanding of human freedom have reopened these issues. The Vatican has developed new, holistic arguments to proscribe reproductive interventions, but critics remain unconvinced that marital relationships and goods have been adequately evaluated. The resolution of this debate will require further experience and reflection.