  Here I review the current state of the field of optical stellar
interferometry, concentrating on ground-based work although a brief report of
space interferometry missions is included. We pause both to reflect on decades
of immense progress in the field as well as to prepare for a new generation of
large interferometers just now being commissioned (most notably, the CHARA,
Keck and VLT Interferometers). First, this review summarizes the basic
principles behind stellar interferometry needed by the lay-physicist and
general astronomer to understand the scientific potential as well as technical
challenges of interferometry. Next, the basic design principles of practical
interferometers are discussed, using the experience of past and existing
facilities to illustrate important points. Here there is significant discussion
of current trends in the field, including the new facilities under construction
and advanced technologies being debuted. This decade has seen the influence of
stellar interferometry extend beyond classical regimes of stellar diameters and
binary orbits to new areas such as mapping the accretion disks around young
stars, novel calibration of the Cepheid Period-Luminosity relation, and imaging
of stellar surfaces. The third section is devoted to the major scientific
results from interferometry, grouped into natural categories reflecting these
current developments. Lastly, I consider the future of interferometry,
highlighting the kinds of new science promised by the interferometers coming
on-line in the next few years. I also discuss the longer-term future of optical
interferometry, including the prospects for space interferometry and the
possibilities of large-scale ground-based projects. Critical technological
developments are still needed to make these projects attractive and affordable.
