Abstract
Interactions between biochemical reactions lie at the heart of functioning of a living cell. In order to formalize these interactions we introduce reaction systems. We motivate them by explicitely stating a number of assumptions/axioms that (we believe) hold for a great number of biochemical reactions – we point out that these assumptions are very different from the ones underlying traditional models of computation. The paper provides the basic definitions, illustrates them by biology and computer science oriented examples, relates reaction systems to some traditional models of computation, and proves some basic properties of reaction systems.