This book is an augmented and revised English translation of the book "Redes de Petri" by Janette Cardoso and Robert Valette, Editora da UFSC, ISBN 85-328-0095-5. Part of this material has been used in graduate and undergraduate courses at ISAE-SUPAERO and INSA.It begins with a discussion about systems and their modeling. After introducing the main definitions about Petri nets, different formal analysis techniques are presented. Interpreted Petri nets, which allow for a representation of the external environment, are introduced in the sequel. All chapters include examples and exercises. A real case example is extensively treated using all the concepts covered.