Professor Yau's work has impacted both mathematics and physics. Calabi-Yau manifolds are among the ‘standard toolkit’ for string theorists today. He has made fundamental contributions at the interface between geometry and theoretical physics. His proof of the positive energy theorem in general relativity demonstrated sixty years after its discovery that Einstein's theory is consistent and stable. His proof of the Calabi conjecture allowed physicists using Calabi-Yau compactification to show that string theory is a viable candidate for a unified theory of nature.