The n-i-p-i doping superlattices, periodic structures composed of n- and p-doped layers, possibly with undoped (i-) regions of the same semi-conductor material in between, form a new class of semiconductors. Apart from their two-dimensional and highly anisotropic electronic structure, a property they share with their compositional superlattice counterparts, they exhibit widely tunable electrical and optical properties that make them very appealing for various applications. Carrier concentration, lumines-cence spectra, absorption coefficient, and refractive index can be modulated by unusually large amounts by weak light signals, injection currents, or external fields. We briefly review the basic theoretical concept and some of the experiments that have confirmed the theory. Then we describe a number of optoelectronic and purely optical device applications. These include wavelength-tunable incoherent and coherent light-emitting devices with high modulation frequency, fast optical modulators, and nonlinear optical devices, possibly exhibiting optical multistability. Although the concept is applicable to any semiconductor that can be doped appropriately, a combination of periodic doping and composition ("hetero n-i-p-i") exhibits particularly appealing potential for devices in the 0.8 to 1.55 µm wavelength range.