The magnetic and ferroelectric properties of multiferroicRMn2O5 (R = Y,Tb, Ho, Er, Tm) are reviewed based on recent neutron diffraction and dielectricmeasurements. Successive phase transitions of magnetic and dielectric ordering were foundto occur simultaneously in this system. The characteristic magnetic ordering ofthe system exhibits an incommensurate–commensurate phase transition, andagain transitions to an incommensurate phase. Special attention is given to themagnetic structure in order to discuss the mechanism for the introduction offerroelectric polarization. For all the compounds examined, the spin configuration forMn4+ and Mn3+ ions in the commensurate magnetic phase, where spontaneous electric polarization occurs,was determined to be a transverse spiral spin structure propagating along thec-axis. By contrast, the alignment of the induced 4f moment ofR3+ ions showed variation, depending on the character of each of the elements. Correspondingresponses to external fields such as a magnetic field, hydrostatic pressure etc at lowtemperature are strongly dependent on the rare earth element present in theRMn2O5 system. The so-called colossal magnetoelectric effect in this system can be easilyinterpreted by the phase transition from the magnetic incommensurate and weakferroelectric phase to the commensurate and ferroelectric phase.