This chapter summarizes some of the results of neutron capture experiments in a fast neutron beam. The aim of this work was to investigate the potential of neutron capture reactions to increase the dose of a fast neutron beam at depth, in the hope that sufficient thermalized neutrons would be present to provide a therapeutic benefit. The experiments were conducted back in the 1980s but are now of interest again. Accelerator-based neutron beams for boron neutron capture therapy (BNCT) allow the energy of the incident neutrons to be modified in a different way than was the case with reactor-based neutron sources. To cover large tumor volumes, it may be necessary to use low-energy fast neutron beams instead of epithermal neutron beams to obtain an adequate dose distribution throughout the entire volume. This chapter also reports some experiments with 6Li and gadolinium. In addition, in animal studies, effective tumor control was achieved by intratumoral injection of 10B-enriched boric acid and intraperitoneal injection of 10B-enriched L-BPA. While no cutaneous side effects occurred after BPA, intratumoral injection of boric acid did. This underscores the importance of targeted uptake of boron carriers into tumor cells for successful BNCT without side effects.