Controllable doping of III–V nanowires grown by the vapor–liquid–solid method remains a challenging task. In sharp contrast to planar layers of the same materials, dopants mainly incorporate into nanowires through a catalyst droplet. We present a thermodynamic theory of the doping process in vapor–liquid–solid III–V nanowires, which provides explicitly the doping level in nanowires versus the nominal doping, material fluxes, and temperature. It is shown that the doping level is directly related to the growth conditions and strongly depends on the epitaxy technique used to grow nanowires. We present and analyze experimental data on p-type Be doping, n-type Te doping, and Si doping of GaAs nanowires that can be either p-type or n-type depending on the growth environment. A good correlation of the model with the data is obtained. We explain why Be and Te doping leads to lower doping levels in GaAs nanowires compared to the nominal ones and how Si doping changes from p-type in molecular beam epitaxy to n-type in hydride vapor-phase epitaxy. These results should be useful for fundamental understanding and developing practical tools for the controllable p-type and n-type doping of GaAs and other III–V nanowires.