Atom interferometers were first realized ten years ago, and since then have
evolved from beautiful demonstrations of quantum physics into instruments
at the leading edge of precision measurement. In this article we trace the
development of atom interferometry, looking at how the physical principles
have been put into practice to achieve ground-breaking experiments. We also
discuss new atom optical techniques that are becoming available and antici
pate the ways in which the consequent improvements will provide new opportu
nities in metrology and the study of fundamental physics.