This volume covers in a comprehensive way and at an elementary level essentially all the theorems and techniques which are commonly used and needed in any branches of mathematics, particularly in complex and in real analytic geometry, in commutative algebra, in algebraic geometry and in real algebraic geometry. In particular it presents Rueckert's complex nullstellensatz, Risler's real nullstellensatz, Tougeron's implicit function theorem and Artin's approximation theorem, to name a few. Up to now a student of any of the subjects mentioned above usually had to learn about power series within the framework of the vast theory of the subject. This had the advantage that he sees many theorems about power series as special instances of more general facts, but on the other hand made it highly time consuming to understand all these theorems. The present book opens another path - one gets acquainted with power series in a direct and elementary way, and then disposes of a good box of tools and examples to penetrate any of the subjects mentioned above, as well as some others.