  Multiferroics - materials which are simultaneously (ferro)magnetic and
ferroelectric, and often also ferroelastic, attract now considerable attention,
both because of the interesting physics involved and as they promise important
practical applications. In this paper I give a survey of microscopic factors
determining the coexistence of these properties, and discuss different possible
routes to combine them in one material. In particular the role of the
occupation of d-states in transition metal perovskites is discussed, possible
role of spiral magnetic structures is stressed and the novel mechanism of
ferroelectricity in magnetic systems due to combination of site-centred and
bond-centred charge ordering is presented. Microscopic nature of multiferroic
behaviour in several particular materials, including magnetite Fe3O4, is
discussed.
