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A new testing system and techniques for measuring the hardness of materials subjected to very high indentation strain rates up to 10⁴ s⁻¹ is presented. This was accomplished by modifying the components of a conventional nanoindentation system with a laser interferometer that records indenter displacements at 1.25 MHz. Using specially developed analysis techniques based on Newton's laws, these displacements can be converted to synchronous measurements of the indentation load, providing for the determination of indentation load–depth curves from which the hardness can be obtained. The testing system has been used to explore the behavior of two materials at opposite ends of the hardness spectrum—hard fused silica and soft single crystalline (111) aluminum. Results show that the hardness of fused silica is relatively insensitive to strain rate, whereas the hardness of aluminum at high strain rates (10³ …