The chapter describes how the band parameters of ternary and quaternary alloys can be interpolated over the entire range of compositions, and tabulate the non-vanishing bowing parameters for most of the common alloys with both zinc-blende and wurtzite lattice structure. It also describes ordering in some of the ternary alloys, and how ordering affects the energy gap. The band parameters of dilute nitrides, dilute bismides, and hexagonal boron nitride are also examined. Finally, the chapter presents schemes for interpolating the optical parameters of III–V alloys, i.e., the real and imaginary parts of the permittivity or dielectric function.