Plasma is the physical state in which most of the visible matter in the universe is found. The properties of plasma are so distinctive that it is often referred to as the fourth state of matter. This book explains the fundamental concepts of plasma physics and provides an introduction to plasma particle dynamics, plasma waves, magnetohydrodynamics, plasma kinetic theory, two-fluid theory and non-linear plasma physics. Emphasis is placed on the underlying physical principles, rather than on mathmetical considerations. The most advanced background knowledge required consists of Maxwell's equations and these are reviewed in the introduction.