Abstract: Monosilicides and monohermanides of transition metals (e.g., MnSi) are characterized by a noncentrosymmetric structure of the B20 type, which leads to the occurrence of the Dzyaloshinski–Moriya (DM) interaction and, as a consequence, helicoidal magnetic ordering. The sign of the DM interaction determines the direction (clockwise or anticlockwise) of magnetic-helicoid twist. It is found that the type of atoms (e.g., Mn and Fe) present in a compound corresponds unambiguously to the sign of DM interaction in both silicides and germanides of transition metals. In turn, the replacement of Mn with Fe atoms in Mn_1-xFe_xGe (and some other) solid solutions results in a flip of the magnetic system chirality. These compounds demonstrates also a complex nature of the first-order temperature phase transition (close to the second-order one), complicated by a multicomponent order parameter and accompanied by a wide range of critical helical fluctuations. The magnetic field–temperature (H–T) phase diagram demonstrates occurrence of a new phase with a skyrmion lattice structure, or A-phase, in a narrow range of fields near the phase-transition point. Application of pressure or replacement of Mn with Fe or Co atoms leads to disappearance of long-range magnetic order and occurrence of a quantum phase transition. This review contains the experimental results obtained using small-angle neutron scattering (SANS) and characterizing the magnetic properties and phase diagrams of helicoidal magnets.