Part of the broader research field of natural computing, Membrane Computing is an area within computing science that aims to abstract computing ideas and models from the structure and functioning of living cells, as well as from the way the cells are organized in tissues or higher order structures. It studies models of computation (known as P systems) inspired by the biochemistry of cells, in particular by the role of membranes in the compartmentalization of living cells into "protected reactors". This handbook provides both a comprehensive survey of available knowledge and established research topics, and a guide to recent developments in the field, covering the subject from theory to applications. The handbook is suitable both for introducing novices to this area of research, and as a main source of reference for active researchers. It sets out the necessary biological and formal background, with the introductory chapter serving as a gentle introduction to and overview of membrane computing. Individual chapters, written by leading researchers in membrane computing, present the state of the art of all main research trends and include extensive bibliographies.