This chapter introduces Petri Nets (PN), the background required to understand Stochastic Petri Nets (SPN). Nowadays, Petri nets refer to a family of models that share common features and were derived the Petri's seminal work. Place-transition nets are a direct bipartite multigraph that is usually defined through sets and matrices, by sets and relations or by sets and bags. Petri nets semantics may be specified in many ways, among them via transition interleaving, simple step, step semantics, and process nets. The adoption of the state equation is an interesting approach for analyzing marking reachability without recurring to individual transition firing. The state equation, however, has its drawbacks. The main issue is related to the mapping of sequences into a vector, since feasible and unfeasible sequences may be mapped into the same vector. Petri nets are formal models. The primary objective of adopting a formalism is its capacity to represent systems neatly.