Glass compositions and high-modulus, and high-strength glass fibers made therefrom, being capable of economical, continuous processing and suitable for the production of high-strength and/or high stiffness, low-weight composites, such as windturbine blades, the glass composition comprises the following constituents in the limits defined below, expressed as weight percentages: between about 56 to about 61 weight percent SiO2; between about 16 to about 23 weight percent Al2O3, wherein the weight percent ratio of SiO2/Al2O3 is between about 2 to about 4; between about 8 to about 12 weight percent MgO; between about 6 to about 10 weight percent CaO, wherein the weight percent ratio of MgO/CaO is between about 0.7 to about 1.5; between about 0 to about 2 weight percent Na2O; less than about 1 weight percent Li2O; and total residual transition metal oxides of less than about 2 weight percent.