  Numerous investigations on the fundamental properties of low-mass stars using
eclipsing binaries indicate a strong discrepancy between theory and
observations that is still awaiting explanation. Current models seem to predict
radii for stars less massive than the Sun that are some 10% smaller than
observed, while their effective temperatures are some 5% larger. Here we
discuss recent new observational data that are relevant to this issue and
review the progress made in understanding the origin of the important
differences with theoretical calculations. Notably, we provide evidence that
stellar activity may be responsible for the mismatch between observations and
theory through two different channels: inhibition of convection or effects of a
significant starspot coverage. The activity hypothesis is put to a test with
observational diagnostics and some of the consequences of the large starspot
coverage are evaluated. We conclude that stellar activity likely plays a key
role in defining the properties of active low-mass stars and that this should
be properly taken into account when investigating young, active stars in
clusters or star-forming regions.
