  I present a brief summary of three different types of binary star -
astrometric, spectroscopic and eclipsing - and tabulate the properties of these
systems that can be determined directly from observations. Eclipsing binary
stars are the most valuable of these, as they are our main source of direct
mass and radius measurements for normal stars. In good cases, masses and radii
can be obtained to better than 1% precision and accuracy using only photometry,
spectroscopy and geometry. These measurements constitute vital empirical data
against which theoretical models of stars can be verified and improved. I give
examples of the use of these systems for constraining stellar theory and the
distance scale, and conclude with a presentation of preliminary results for the
solar-type eclipsing binary 1SWASP J034114.25+201253.5.
