From flat-panel televisions to thermonuclear fusion for energy production, plasmas currently have numerous and wide applications in sciences and industry. A diversity of plasma diagnostics is available to physicists and engineers to measure and control plasma parameters. Among them, the Langmuir probe is the most inexpensive and most popular instrument and method. The Langmuir probe is a small electrode which is submerged in plasma in order to measure the probe current-voltage characteristic. The same characteristic is processed further to derive the electron and ion concentration, the electron distribution function, and the plasma potential at the probe location. Langmuir probe diagnostics afford rapid measurements of the electron distribution function and plasma potential at a good time resolution, ~ 10-8 seconds in a wide range of plasma densities 10+3 - 10+14 cm-3, and the electron energy from the room temperature to hundreds of electron-volts - qualities which are essential for researchers. In view of these facts, Langmuir probe diagnostics are applied very frequently to measuring plasma parameters. This book will be useful in teaching plasma diagnostics to undergraduate and graduate students in plasma physics courses. And it will also serve as a practical reference manual for physicists and engineers working in the growing area of plasma physics. The reader of this book will learn what kind of plasma parameters the Langmuir probe can measure, how to develop the probe diagnostics for specific cases, and how the probe data obtained should be processed to deduce reliable plasma parameters. In this book, the reader can find not only the basic physics information important to understanding the principles of probe operation, but also how the "real" probe disturbs plasma, and how it is possible to reconstruct undisturbed plasma parameters with available probe data.