  Recent studies suggest the existence of very massive stars (VMS) up to 300
solar masses in the local Universe. As this finding may represent a paradigm
shift for the canonical stellar upper-mass limit of 150 solar masses, it is
timely to evaluate the physics specific to VMS, which is currently missing. For
this reason, we decided to construct a book entailing both a discussion of the
accuracy of VMS masses (Martins), as well as the physics of VMS formation
(Krumholz), mass loss (Vink), instabilities (Owocki), evolution (Hirschi), and
fate (theory -- Woosley & Heger; observations -- Smith).
