The origin of magnetic hardening has been examined in Fe-Nd-B magnets by correlating the magnetic properties with the microstructure and magnetic domain structure observed with electron microscopy. Hysteresis loops and susceptibility studies show a magnetic anomaly at around 140 K. Microstructure studies reveal the presence of at least four phases with compositions Fe14R2B, Fe4NdB4, a high-Nd-content phase, and α-Fe. Initial magnetization studies, magnetic viscosity data, the temperature dependence of coercivity, and magnetic domain studies all indicate domain wall pinning at grain boundaries.