In this book, we have addressed three aspects of PN research: radiation mechanisms, evolution, and applications. We have discussed the various physical mechanisms that are responsible for the emission of radiation from PN, and how different techniques (imaging, photometry and spectroscopy) in the radio, submillmeter, infrared, optical, ultraviolet, and X-ray can be used to probe the physical conditions in different parts of the nebulae. The wealth of data obtained through multi-wavelength observations have served as laboratories for the testing of radiation theories as well as for atomic and molecular physics. The discoveries of new phenomena such as forbidden lines and unidentified infrared features have stimulated the laboratory spectroscopy of atomic and molecular species.

The central stars of PN are hot and luminous objects. Although most of the starlight is in the form of high-energy UV photons, the nebulae are able to intercept these photons and downgrade them to visible wavelengths. Through photoionization, the radiative energy of the star is transferred to the kinetic energy pool of the gaseous nebula, which then emits low-energy line and continuum photons through the processes of recombination and collisional excitation. As the result, PN become bright visible objects and can be detected at large (cosmological) distances. For nearby PN, it is possible to obtain very high-quality spectra which allow for the accurate determination of chemical abundances and kinematic structure of the nebulae.