We have considered chiral-achiral phase transitions that can occur in
some liquid crystals. This study deals with such transformations induc
ed by external electric and magnetic fields. We discuss phase changes
mediated by disclinations, single solitons, soliton lattices and those
occurring without any defect. Some unusual aspects of these transitio
ns such as reentrance, occurrence of tricritical points and Lifshitz p
oints have been highlighted.