We study the electronic properties of native defects and intentional dopant impurities in MoO3, a widely used transparent conductor. Using first-principles hybrid functional calculations, we show that electron polarons can be self-trapped, but they can also bind to defects; thus, they play an important role in understanding the properties of doped MoO3. Our calculations show that oxygen vacancies can cause unintentional n-type doping in MoO3. Mo vacancies are unlikely to form. Tc and Re impurities on the Mo site and halogens (F, Cl, and Br) on the O site all act as shallow donors but trap electron polarons. Fe, Ru, and Os impurities are amphoteric and will compensate n-type MoO3. Mn dopants are also amphoteric, and they show interesting magnetic properties. These results support the design of doping approaches that optimally exploit functionality.