  The symmetry properties, order parameters, and magnetoelectric phase diagrams
of multiferroics are discussed. After brief reviews of Ni$_3$V$_2$O$_8$,
TbMnO$_3$, and RbFe(MoO$_4$)$_2$, we present a detailed analysis of
RMn$_2$O$_5$ (with R=Y, Ho, Dy, Er, Tb, Tm).
