The idea that planetary nebulae (PN) originate from the outer atmospheres of red giant was first proposed by Shkloskii (1956). In the early 1970’s, it was generally believed that PN were created by a sudden envelope ejection; however, the exact nature of the instability responsible for the ejection was not understood. Furthermore, it was not clear how the observed masses of PN are related to the amount of mass ejected, and there was no explanation of how the observed shell morphologies of PN could be created by such instabilities. From the observed sizes and expansion velocities of PN, we know that the dynamical lifetime of a PN is ~104yr. During this period, the star has to evolve from the red side of the H-R diagram, where the envelope is ejected, to the blue side of the H-R diagram, where the stellar temperature is high enough to photoionize the ejecta. If the star evolves too slowly, the nebula will have dispersed into the interstellar medium before it is ionized. If the star evolves too fast, then the nebula will only be illuminated for a short time before the stellar luminosity drops on the white dwarf cooling track (Renzini 1982). It is therefore surprising that PN is such a common phenomenon, with ~30000 PN estimated to be in the Galaxy now.