Glide and climb phenomena ofa⟨100⟩ loops and dislocations in magnesium oxide have been investigated using electron microscope techniques. It has been clearly demonstrated thata⟨100⟩ dislocations could glide along ⟨100⟩ directions and from these observations the lattice friction stress was estimated to be 1.0±0.1×109dyn cm−2. From the observed shrinkage rates of loops as a function of temperature, a value of 4.5±0.3 eV for the activation energy for self‐diffusion was obtained. This study had also shown thata⟨100⟩ dislocations can split into, or be formed from, the more commonly observeda/2⟨110⟩ dislocations in magnesium oxide.