A curable polymer composition comprising a block copolymer suitable for toughening a thermoset resin and one or more uncured thermoset resin precursor(s) of said thermoset resin, said block copolymer having at least one block derived from a thermoplastic aromatic polymer wherein an aromatic radical lies within the polymer backbone of the thermoplastic aromatic polymer and wherein the thermoplastic aromatic polymer exhibits a glass transition temperature (Tg) of at least 150°C, and at least one block derived from a low Tg polymer wherein: (i) the low T g polymer exhibits a Tg in the range of from -130°C to +40 °C; (ii) the aromatic polymer is soluble in the uncured thermoset resin precursor(s), (iii) the low Tg polymer is insoluble in the uncured thermoset resin precursor, and wherein the aromatic polymer comprises one or more polyarylsulphone(s) comprising ether-linked repeating units, optionally further comprising thioether-linked repeating units, the units being selected from: -[ArSO 2 Ar] n - and optionally from: -[Ar] a - wherein: Ar is phenylene; n = 1 to 2 and can be fractional; a = 1 to 3 and can be fractional and when a exceeds 1, said phenylene groups are linked linearly through a single chemical bond or a divalent group other than -SO 2 -, or are fused together, provided that the repeating unit -[ArSO 2 Ar] n - is always present in the polyarylsulphone in such a proportion that on average at least two of said - [ArSO 2 Ar] n - units are in sequence in each polymer chain present, wherein the polyarylsulphone has one or more reactive pendant and/or end group(s) selected from OH and NH 2; and wherein the aromatic polymer is a polymer wherein the mass fraction of aromatic diradicals that are linked together by the said linking groups in the polymer is at least 51 %; and further wherein the low Tg polymer is selected from saturated aliphatic polyesters derived from at least divalent linear, branched or cyclic aliphatic alcohols having from 2 to 60 carbon atoms, and at least divalent linear, branched or cyclic aliphatic carboxylic acids having from 3 to 60 carbon atoms, with the provisos that at least one of the alcohol or the acid components has at least 4 carbon atoms and if the saturated aliphatic polyester is derived from more than one alcohol and/or more than one acid, the average number of carbon atoms in the alcohol or acid component is at least 4. (i) the low T g polymer exhibits a Tg in the range of from -130°C to +40 °C; (ii) the aromatic polymer is soluble in the uncured thermoset resin precursor(s), (iii) the low Tg polymer is insoluble in the uncured thermoset resin precursor, and wherein the aromatic polymer comprises one or more polyarylsulphone(s) comprising ether-linked repeating units, optionally further comprising thioether-linked repeating units, the units being selected from: -[ArSO 2 Ar] n - and optionally from: -[Ar] a - wherein: Ar is phenylene; n = 1 to 2 and can be fractional; a = 1 to 3 and can be fractional and when a exceeds 1, said phenylene groups are linked linearly through a single chemical bond or a divalent group other than -SO 2 -, or are fused together, provided that the repeating unit -[ArSO 2 Ar] n - is always present in the polyarylsulphone in such a proportion that on average at least two of said - [ArSO 2 Ar] n - units are in sequence in each polymer chain present, wherein the polyarylsulphone has one or more reactive pendant and/or end group(s) selected from OH and NH 2; and wherein the aromatic polymer is a polymer wherein the mass fraction of aromatic diradicals that are linked together by the said linking groups in the polymer is at least 51 %; and further wherein the low Tg polymer is selected from saturated aliphatic polyesters derived from at least divalent linear, branched or cyclic aliphatic alcohols having from 2 to 60 carbon atoms, and at least divalent linear, branched or cyclic aliphatic carboxylic acids having from 3 to 60 carbon atoms, with the provisos that at least one of the alcohol or the acid components has at least 4 carbon atoms and if the saturated aliphatic polyester is derived from more than one alcohol and/or more than one acid, the average number of carbon atoms in the alcohol or acid component is at least 4. Ar is phenylene; n = 1 to 2 and can be fractional; a = 1 to 3 and can be fractional and when a exceeds 1, said phenylene groups are linked linearly through a single chemical bond or a divalent group other than -SO 2 -, or are fused together, provided that the repeating unit -[ArSO 2 Ar] n - is always present in the polyarylsulphone in such a proportion that on average at least two of said - [ArSO 2 Ar] n - units are in sequence in each polymer chain present, wherein the polyarylsulphone has one or more reactive pendant and/or end group(s) selected from OH and NH 2; and wherein the aromatic polymer is a polymer wherein the mass fraction of aromatic diradicals that are linked together by the said linking groups in the polymer is at least 51 %; and further wherein the low Tg polymer is selected from saturated aliphatic polyesters derived from at least divalent linear, branched or cyclic aliphatic alcohols having from 2 to 60 carbon atoms, and at least divalent linear, branched or cyclic aliphatic carboxylic acids having from 3 to 60 carbon atoms, with the provisos that at least one of the alcohol or the acid components has at least 4 carbon atoms and if the saturated aliphatic polyester is derived from more than one alcohol and/or more than one acid, the average number of carbon atoms in the alcohol or acid component is at least 4.