A polymer polyol composition having a solids content of from 10 to 72% by weight, and comprising a polymer polyol having a solids content of from 44 to 55 % by weight, a viscosity at 25oC of less than 50,000 mPa.s, and which comprises the reaction product of: at least one base polyol having a functionality of from 2 to 8 and a hydroxyl number of from 20 to 400; one or more ethylenically unsaturated monomers; a preformed stabilizer which comprises the reaction product of: (a) a macromer that contains reactive unsaturation and comprises the reaction product of: (i) a starter compound having a functionality of 2 to 8 and a hydroxyl number of from 20 to 50; (ii) 0.1 to 3% by weight, based on 100% by weight of the sum of components (i), (ii) and (iii), of a hydroxyl-reactive compound that contains reactive unsaturation; and (iii) 0.05 to 2.5% by weight, based on 100% by weight of the sum of components (i), (ii) and (iii), of diisocyanate; with (b) one or more ethylenically unsaturated monomers; and (c) at least one free radical initiator; in the presence of (d) a polymer control agent; and, optionally, (e) a liquid diluent; in the presence of at least one free radical initiator; and, optionally, a chain transfer agent; and at least one polyol component having a functionality of from 2 to 6 and a hydroxyl number of from 25 to 200; wherein component is present in an amount sufficient to reduce the total solids content in said polymer polyol by at least 5% by weight; wherein component (a)(ii) the hydroxyl-reactive compound that contains reactive unsaturation is methyl methacrylate, ethyl methacrylate, maleic anhydride, isopropenyl dimethyl benzyl isocyanate, 2-isocyanatoethyl methacrylate, adducts of isophorone diisocyanate and 2-hydroxyethyl methacrylate, adducts of toluenediisocyanate and 2-hydroxypropyl acrylate. a polymer polyol having a solids content of from 44 to 55 % by weight, a viscosity at 25oC of less than 50,000 mPa.s, and which comprises the reaction product of: at least one base polyol having a functionality of from 2 to 8 and a hydroxyl number of from 20 to 400; one or more ethylenically unsaturated monomers; a preformed stabilizer which comprises the reaction product of: (a) a macromer that contains reactive unsaturation and comprises the reaction product of: (i) a starter compound having a functionality of 2 to 8 and a hydroxyl number of from 20 to 50; (ii) 0.1 to 3% by weight, based on 100% by weight of the sum of components (i), (ii) and (iii), of a hydroxyl-reactive compound that contains reactive unsaturation; and (iii) 0.05 to 2.5% by weight, based on 100% by weight of the sum of components (i), (ii) and (iii), of diisocyanate; with (b) one or more ethylenically unsaturated monomers; and (c) at least one free radical initiator; in the presence of (d) a polymer control agent; and, optionally, (e) a liquid diluent; in the presence of at least one free radical initiator; and, optionally, a chain transfer agent; and at least one base polyol having a functionality of from 2 to 8 and a hydroxyl number of from 20 to 400; one or more ethylenically unsaturated monomers; a preformed stabilizer which comprises the reaction product of: (a) a macromer that contains reactive unsaturation and comprises the reaction product of: (i) a starter compound having a functionality of 2 to 8 and a hydroxyl number of from 20 to 50; (ii) 0.1 to 3% by weight, based on 100% by weight of the sum of components (i), (ii) and (iii), of a hydroxyl-reactive compound that contains reactive unsaturation; and (iii) 0.05 to 2.5% by weight, based on 100% by weight of the sum of components (i), (ii) and (iii), of diisocyanate; with (b) one or more ethylenically unsaturated monomers; and (c) at least one free radical initiator; in the presence of (d) a polymer control agent; and, optionally, (e) a liquid diluent; in the presence of (a) a macromer that contains reactive unsaturation and comprises the reaction product of: (i) a starter compound having a functionality of 2 to 8 and a hydroxyl number of from 20 to 50; (ii) 0.1 to 3% by weight, based on 100% by weight of the sum of components (i), (ii) and (iii), of a hydroxyl-reactive compound that contains reactive unsaturation; and (iii) 0.05 to 2.5% by weight, based on 100% by weight of the sum of components (i), (ii) and (iii), of diisocyanate; with (i) a starter compound having a functionality of 2 to 8 and a hydroxyl number of from 20 to 50; (ii) 0.1 to 3% by weight, based on 100% by weight of the sum of components (i), (ii) and (iii), of a hydroxyl-reactive compound that contains reactive unsaturation; and (iii) 0.05 to 2.5% by weight, based on 100% by weight of the sum of components (i), (ii) and (iii), of diisocyanate; with (b) one or more ethylenically unsaturated monomers; and (c) at least one free radical initiator; in the presence of (d) a polymer control agent; and, optionally, (e) a liquid diluent; in the presence of at least one free radical initiator; and, optionally, a chain transfer agent; and at least one polyol component having a functionality of from 2 to 6 and a hydroxyl number of from 25 to 200;