First-generation diode lasers produced stimulated emission from thick active regions consisting of a single semiconductor material. However, not long after epitaxial growth techniques advanced to the point of allowing thin heterostructure layers to be deposited with a high degree of control and precision, the quantum well laser was born. It soon became apparent that besides providing a valuable vehicle for studying fundamental quantum physics and optics in a solid state environment, quantum well diode lasers were also capable of dramatic performance improvements over their bulk counterparts. The lowering of threshold current densities was especially impressive.