We review our recent work in the field of molecule-based magnets showing the structural and magnetic properties of a special class of hybrid organic–inorganic compounds, i.e. metal(II) organophosphonates. The synthesis, the crystal structures and, in particular, the magnetic studies of selected examples of compounds of formulas M(II)[(R–PO3)(H2O)], and M2[(O3P–R′–PO3)(2H2O)] M=Cr, Fe, Co; R=C n H2n+1, n =1, 2, 3… and C6H5, R′=(CH2)2 prepared in our laboratory are presented and discussed. Metal alkylphosphonates, except those of Co(II), are weak ferromagnets at low temperatures. The observed magnetic ordering temperature T N varies from 4.2 to 25K, depending on the transition metal ions and on crystal and molecular structure. Moreover, in the case of a bifunctional molecule like aminoethylphosphonic acid, NH2(CH2)2PO3H2, or the carboxyethylphosphonic acid, HO2C(CH2)2PO3H2, is used as a ligand, then a novel Cr(II) compound of formula Cr[NH3(CH2)2PO3(Cl)(H2O)] and a microporous Fe(III) salt (NH4)[Fe2(OH){O3P(CH2)2CO2}2] are isolated. The latter are both polar and, more interesting, Cr(II) ammoniumethylphosphonate chloride results to be a weak ferromagnet below T N =5.0K.