The phenomena observed in young stellar objects (YSO), circumstellar regions and extra-galactic nuclei show some similarity in their morphology, dynamical and physical processes, though they may differ in scale and energy.

The European Workshop on Astronomical Molecules 2004 gave astronomers a unique opportunity to discuss the links among the observational results and to generate common interpretations of the phenomena in stars and galaxies, using the available diagnostic tools such as masers and dense molecular gas. Their theoretical understanding involves physics, numerical simulations and chemistry.

Including a dozen introductory reviews, topics of papers in this book also cover: maser and dense gas diagnostics and related phenomena, evolution of circumstellar regions around protostars, evolution of circumnuclear regions of active galaxies, diagnostics of the circumnuclear gas in stars and galactic nuclei.

This book summarizes our present knowledge in these topics, highlights major problems to be addressed, and finally proposes future work.