Masses, radii and luminosities of distant stars can only be measured accurately in eclipsing binaries. The most massive eclipsing binary currently known is WR 20a, which consists of two ~ 80 M⊙ stars in a 3.7 d orbit. Analogs of WR 20a are bound to exist both in massive stellar clusters in our Galaxy and in nearby galaxies. The nearest ones are located in the clusters near the Galactic Center: the Center, Arches, and Quintuplet clusters. The severe amount of reddening in the galactic disk makes the study of galactic clusters challenging. However, with current 8-m class telescopes, the study of massive stars in nearby galaxies is also feasible. The nearest Local Group galaxies (LMC, SMC, M 31, M 33) provide the perfect laboratory for studying massive stars and determining their properties as a function of metallicity. Such studies will constrain models, confirm the dependence of evolution on metallicity and help understand the rate and nature of supernovae and gamma-ray bursts.