The concept of a membrane system (a P system) was introduced by Gheorghe Pǎun in 1998 [9,10], with the aim of formulating a computational device abstracted from the architecture and the functioning of the living cell. Since that time, the theory of membrane systems has proved to be a successful area in bio-inspired computing. The main ingredient of a P system is a hierarchically embedded structure of membranes. Each membrane encloses a region that contains objects and might also contain other membranes. The outmost membrane is called the skin membrane. There are rules associated to the regions describing the evolution of the objects which represent chemical substances. The evolution rules correspond to chemical reactions, and the evolution of the system to a computation. The main features of P systems include the transformation (rewriting) of objects, their moving among the different regions (communication), and possibly other additional capabilities such as, for example, a dynamically changing membrane structure, or special constraints added to the sets of rules. At any moment in time, the membrane system can be described by its configuration (its state) which consists of the actual membrane structure and the contents of the regions.