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Twin and grain boundaries impede the motion of magnetic domain walls in the orthoferrites. Bubbles distort and collapse before it is possible to propagate them through twin boundaries and all but very low angle grain boundaries. A new experimental technique has been used to measure the coercivity of isolated defects in nearly perfect crystals. The coercivity measured is the minimum applied field required to force a planar domain wall past a defect. Models of the spin configurations near twin and grain boundaries in the presence and absence of magnetic domain walls have been developed to explain the observed coercivities.