The efficiency of neutral beam heating and current drive depends crucially on the deposition of the energy and momentum of the beam in the plasma. This deposition is determined by the atomic processes involved in the stopping (or effective ionization) of the neutral beam atoms. These processes have been studied in detail for the energy range from 10 keV/u to 10 MeV/u. The processes considered include both the ground state and the excited state of the beam atoms, thus allowing for the multistep ionization of the beam in collisions with the plasma constituents and impurities. The effective beam stopping cross-section has been calculated for a wide variety of beam and plasma parameters. The atomic database necessary for these calculations has been documented using the best data available at present. The stopping cross-section data are also given in terms of a convenient analytic fit, which can be used either in computer calculations or for simpler analytic estimates of neutral beam penetration.