Reaction systems is a model of computation inspired by the functioning of the living cell and, in particular, by the interactions between biochemical reactions in the living cell. A reaction is specified on an abstract level through its set of reactants, its set of inhibitors, and its set of products. A reaction is enabled if all of its reactants and none of its inhibitors are available. The key concept in this model is that the interactions are based on the mechanisms of facilitation and inhibition: the result of a reaction may facilitate some reactions (by producing their reactants) and it may inhibit some reactions (by producing their inhibitors). A reaction system consists of a finite set of reactions. The interactions between its reactions, together with the interactions with the environment (explicitly included in the model) define the dynamic behavior/computation of the system. An important feature of reaction systems (motivated by the basic bioenergetics of the living cell) is their non-permanency: every entity produced by a reaction is only available for a specific part of the computation, unless it is re-introduced (sustained) by a reaction enabled in the computation.