Jets are amongst the most mysterious phenomena to be discovered in modern astronomy. They are able to form and propagate under almost all conditions associated with a vast range of astrophysical objects. This book is concerned with all the diverse jets which have so far been found beyond our own planet. It will be seen that our universe is replete with jets because they act as essential outlets or valves for regulating the birth and early development of discrete objects and their extended environments.

The purpose here is to assimilate all we know from the different disciplines in which they are encountered. I cannot try to review radio galaxies, star formation, comets or planetary nebula, but only the parts in which jets are essential to their understanding. We thus learn about the driving mechanisms involved and their consequent impact, and so learn to appreciate the diversity. The idea is to accumulate, perhaps possible for the last time, all the material which relates to the phenomenom referred to as jets. Hence this is not a series of reviews but a gathering of essential knowledge. And, consequently, by establishing their common properties, this book hopes to represent a turning point in what we have come to understood as jets and what we will go on to discover.

It will be attempted to make this book self-contained with a modicum of required knowledge. It should serve as a timely introduction for astronomy students who seek to develop a broad approach to understand the ‘bigger picture’.