Optical and electronic properties of semiconductors are strongly influenced by the different possibilities of carriers to be distributed among the various extrema of the band structure or the transfer between them. The monograph Optical Properties of III-V Semiconductors is concerned with the III-V bulk and low-dimensional semiconductors with the emphasis on the implications of multi-valley bandstructures on the physical mechanisms essential for opto-electronic devices. The optical response of such semiconductor materials is determined by many-body effects like screening, gap narrowing, Fermi-edge singularity, electron-hole plasma and liquid formation. The discussion of the latter features is presented self-consistently with the dynamics of excitons and carriers resulting from intervalley coupling.