The interaction between lattice imperfections and ordered spin structures gives rise to the following effects: 1. Modification of the domain structure. 2. Pinning of the domain walls. 5. Reduction of the nucleation field. The theoretical backgrounds for the treatment of the long range and short range magnetic interactions is developed within the framework of micromagnetism and of the Heisenberg model. Special topics to be considered are: a) The pinning of very narrow domain walls by point defects dislocations and antiphase boundaries. b) The pinning of bubble domains. c) The effect of dislocations on the nucleation field of high coercivity materials.