We address the attainability of the Hashin-Shtrikman bounds for multiphase composite materials. We demonstrate that the Hashin-Shtrikman bounds are not always attainable and give new restrictions on the attainable Hashin-Shtrikman bounds in terms of the conductivities and volume fractions of the constituent phases. New optimal microstructures are also constructed to attain the Hashin-Shtrikman ...