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Metallic glasses exhibit high hardness values, typified by a plastic constraint factor (the ratio of hardness to compressive yield strength) that is considerably larger than that seen in ductile crystalline metals. The reason for this behaviour is investigated in this paper, by conducting a combined experimental and numerical study on the spherical indentation response of a Zr-based bulk metallic glass (Vitreloy-1). An extended Drucker–Prager constitutive theory with different levels of pressure sensitivity is employed in the numerical simulations. In addition, a modified version of the expanding cavity model, which incorporates pressure sensitivity of yielding, is used to interpret the trends exhibited by the experimental and computational results. Attention is focused on the development of plastic flow beneath the indenter as well as the variation of hardness with indentation strain. It is shown that the high plastic constraint factor exhibited by metallic glasses at large indentation strains is an outcome of their pressure sensitive plastic deformation. These observations are discussed in the context of an appropriate constitutive model for yielding of metallic glasses. [Copyright &y& Elsevier]