Publisher Summary This chapter discusses the measurements of charge transfer and ionization in collisions involving hydrogen atoms. The modulated crossed-beam technique is designed to utilize low-density beams of highly dissociated hydrogen effusing from a small aperture in either a simple gas discharge tube or a tungsten tube furnace. Most measurements are carried out using tungsten tube furnace sources which conveniently provide ground-state H-atom beams with a dissociation fraction of at least 90%. The primary ion beam is arranged to intersect (at right angles) the H-atom beam in a high-vacuum region. Measurements of the yield of slow ions or electrons from the crossed-beam intersection region using parallel-plate collectors and/or mass analysis of the slow ions enable cross sections for charge transfer or ionization to be determined. The signals from the process of interest are generally much smaller than those arising from interactions of the primary beam with the background gas. By mechanically chopping the H-atom beam at a fixed frequency, the required signal can be selectively distinguished by its specific frequency and phase. Cross sections for electron capture into specific excited states may, in principle, be determined by recording the yield of photons emitted in the spontaneous decay of the excited collision products formed in the crossed-beam region.