Net theory is a theory of systems organization which had its origins,
about 20 years ago, in the dissertation of C. A. Petri [1]. Since this
seminal paper, nets have been applied in various areas, at the same time
being modified and theoretically investigated. In recent time, computer
scientists are taking a broader interest in net theory. The main concern
of this book is the presentation of those parts of net theory which can
serve as a basis for practical application. It introduces the basic net
theoretical concepts and ways of thinking, motivates them by means of
examples and derives relations between them. Some extended examples il-
lustrate the method of application of nets. A major emphasis is devoted
to those aspect which distinguish nets from other system models. These
are for instance, the role of concurrency, an awareness of the
finiteness of resources, and the pos- sibility of using the same
representation technique of different levels of ab- straction. On
completing this book the reader should have achieved a system- atic
grounding in the subject allowing him access to the net literature
[25]. These objectives determined the subjects treated here. The
presentation of the material here is rather more axiomatic than in-
ductive. We start with the basic notions of 'condition' and 'event' and
the con- cept of the change of states by (concurrently) occurring
events. By generali- zation of these notions a part of the theory of
nets is presented.