  In this contribution we present the properties of the most massive stars
known by the end of 2013. We start with a summary of historical claims for
stars with masses in excess of several hundreds, even thousands of solar
masses. We then describe how we determine masses for single stars. We focus on
the estimates of luminosities and on the related uncertainties. We also
highlight the limitations of evolutionary models used to convert luminosities
into masses. The most luminous single stars in the Galaxy and the Magellanic
Clouds are subsequently presented. The uncertainties on their mass
determinations are described. Finally, we present binary stars. After recalling
some basics of binary analysis, we present the most massive binary systems and
the estimates of their dynamical masses.
