Accurate determinations of masses and radii in binary stars , along with estimates of the effective temperatures , metallicities , and other properties , have long been used to test models of stellar evolution .
As might be expected , observational constraints are plentiful for main-sequence stars , although some problems with theory remain even in this regime .
Models in other areas of the H-R diagram are considerably less well constrained , or not constrained at all .
I summarize the status of the field , and provide examples of how accurate measurements can supply stringent tests of stellar theory , including aspects such as the treatment of convection .
I call attention to the apparent failure of current models to match the properties of stars with masses of 1.1–1.7 M _ { \odot } that are near the point of central hydrogen exhaustion , possibly connected with the simplified treatment of convective core overshooting .