In this talk, the mechanical behavior of BCC high-entropy alloys within the Ti-Zr-Hf-Nb-Ta system will be analyzed and discussed. These are the only refractory BCC high-entropy alloys that are malleable at room temperature. They can therefore be deformation processed and recrystallized to rigorously control microstructure. They are also relatively ductile at room temperature, which allows tensile tests to be performed, for example, as a function of temperature and strain rate. Model pseudo-binaries within this system are tested and their deformation and fracture behavior correlated with chemical composition. The observed effects of chemical composition are rationalized in terms of elementary deformation mechanisms, which in turn depend on fundamental physical and mechanical properties (e.g., melting point, shear modulus) and the thermodynamic driving forces for phase transformations. Research sponsored by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.