This book is intended to dispel the notion that collective phenomena are an obscure and inaccessible topic. Having become increasingly important in modern storage rings, collective effects deserve to become part of the general education in accelerator physics. I tried to write the book I was looking for when I started to study collective instabilities: an introduction that can be read from cover to cover with m- erate effort and familiarizes the reader with the basic concepts. Self-contained appendices contain what I considered worth discussing but might impair the readability of the main text, such as a reminder of basic relations and some background information on selected topics. Finally, an extensive list of r- erences was compiled (with some emphasis on accessibility of the source) in order to facilitate the search for more detailed information. A previous version of the present text was submitted in German language as Habilitationsschrift – a postdoctoral thesis, which is a peculiarity of the German academic system – to the Humboldt University of Berlin. It grew from my occupation with collective phenomena over ten years at the thi- generation synchrotron light source BESSY II. This has some consequences regarding the character of this text: • There is a clear emphasis on synchrotron light sources and electron storage rings. However, many concepts (wake field, impedance, beam spectrum, beam lifetime, etc.) are quite general. • The selection of material reffects my personal experience and some readers may want to skip over details that I considered important, while others may find that their favorite topic was not covered in sufficient detail.