This book provides a comprehensive summary of studies of the Z boson in electron positron interactions. The results that have been obtained have achieved unprecedented accuracy and have firmly established the Electroweak Standard Model as the cornerstone of our current understanding of modern particle physics. The book introduces the background to the Standard Model and the role of the Z boson and describes briefly the accelerators and experiments involved in these results. The five main chapters deal with the detailed measurements of the electroweak parameters of the Z, the study of QCD, heavy quark physics, tau lepton physics and the search for new particles. Each chapter ends with a summary and outlook to prospects of future accelerators. Most of the results are from the first six years running at the first phase of electron positron collider, LEP, at energies corresponding to the Z mass, but results from the SLAC Linear Collider and some new results at higher energies are also included.