This paper presents the results of recent investigations into the aerodynamics of simulated runback ice accretion on airfoils. Aerodynamic testing was performed on a full-scale, 72-in.-chord (1828.8-mm-chord), NACA 23012 airfoil model over a Reynolds number range of 4.7 x 10⁶ to 16.0 x 10⁶ and a Mach number range of 0.10 to 0.28. A high fidelity ice-casting simulation of a runback ice accretion was attached to the model leading edge. For Re = 16.0 x 10⁶ and M = 0:20, the artificial ice shape decreased the maximum lift coefficient from 1.82 to 1.51 and decreased the stalling angle of attack from 18.1 to 15.0 deg. In general, the iced-airfoil performance was insensitive to Reynolds and Mach number changes over the range tested. Aerodynamic testing was also conducted on a quarter-scale NACA 23012 model [18 in. (457.2 mm) chord] at Re = 1:8 x 10⁶ and M = 0:18, using low-fidelity geometrically scaled simulations of the full-scale casting. It was found that simple two-dimensional simulations of the upper- and lower surface runback ridges provided the best representation of the full-scale, high-Reynolds-number, iced-airfoil aerodynamics. Higher-fidelity simulations of the runback ice accretion that included geometrically scaled three-dimensional features resulted in larger performance degradations than those measured on the full-scale model. Based upon this research, a new subclassification of spanwise-ridge ice is proposed that distinguishes between short and tall ridges. This distinction is made in terms of the fundamental aerodynamic characteristics as described in this paper.