Petri nets which are used for modeling and analyzing complex systems that can be characterized as synchronous, parallel, simultaneous, distributed, resource sharing, nondeterministic and/or stochastic form a powerful modeling tool and are widely used today. In this study, fundamental concepts of Petri nets and their extensions are presented. Since the application area of Petri nets is wide, the subject is handled in the view of flexible manufacturing systems. A two stage modeling approach which combines the modeling power of stochastic Petri nets together with fuzzy sets is also presented. A numerical example is given to present how the proposed approach can be applied. We believe that this approach better represents both dimensions of uncertainty, stochastic variability and imprecision, in system modeling.