Laser physics and nonlinear optics are fields which have been intimately con- nected from their beginning. Nonlinear optical effects such as second-har- monic generation fulfil vital functions in many laser systems. Conversely advances in laser development quickly lead to progress in nonlinear optics. Of particular importance has been the development of tunable visible and uv lasers. With the ability to tune the laser frequency into close resonance with atomic transition frequencies, one can produce a large resonance en- hancement of the nonlinearity. This permits the observation of a great...