  For many decades the determination of accurate fundamental parameters for
stars (masses, radii, temperatures, luminosities, etc.) has mostly been the
domain of eclipsing binary systems. That has begun to change as long-baseline
interferometric techniques have improved significantly, and powerful new
instruments have come online. This paper will review the status of the field,
and in particular how the knowledge of precise stellar properties helps us
understand stars. Main-sequence stars similar to the Sun are by far the best
studied, but much remains to be done for other kinds of objects such as
early-type as well as late-type stars including brown dwarfs, evolved stars,
metal-poor stars, and pre-main sequence stars. Progress is illustrated with
several examples of how interferometry has contributed significantly in some of
these areas.
