Michael Hart has seized the opportunity presented by the manufacture of highly perfect crystals for the semiconductor industry to develop entirely new techniques in X-ray optics, with great advances in sensitivity and precision. In 1965, with Ulrich Bonse, he originated X-ray interferometry. He has developed applications thereof to metrology for distances ranging from micrometres to picometres. He has made absolute precision determinations of lattice parameter, structure factors, and dispersion corrections for silicon and germanium. From these follow a redetermination of Avogadro’s number and improved delineation of electron density in these crystals. He has developed methods employing multiple reflection (and multiple sources) which permit intercomparison of lattice spacing between various crystals in 1 part to 109. His very sensitive qualitative and quantitative tests for crystal perfection and lattice parameter variation give information on residual impurities important for the semiconductor industry, and have enabled him to make substantial improvements in instruments and technique for X-ray and gamma-ray spectroscopy. He received, in 1970, jointly with Ulrich Bonse, the American Crystallographic Association’s B. E. Warren Award for Diffraction Physics.