  Stellar fundamental properties (masses, radii, effective temperatures) can be
extracted from observations of eclipsing binary systems with remarkable
precision, often better than 2%. Such precise measurements afford us the
opportunity to confront the validity of basic predictions of stellar evolution
theory, such as the mass-radius relationship. A brief historical overview of
confrontations between stellar models and data from eclipsing binaries is
given, highlighting key results and physical insight that have led directly to
our present understanding. The current paradigm that standard stellar evolution
theory is insufficient to describe the most basic relation, that of a star's
mass to its radius, along the main sequence is then described. Departures of
theoretical expectations from empirical data, however, provide a rich
opportunity to explore various physical solutions, improving our understanding
of important stellar astrophysical processes.
