This chapter will aim to present good practice in, and the benefits of, formalization in modeling multiagent systems (MAS). To achieve this, the authors will first reiterate the usefulness of modeling systems, while placing the paradigms associated with a multiagent approach in context. Then, they will argue that the use of graphic modeling languages enhances the exchanges between the parties involved in the design of an MAS. Following this, two types of graphic models based on the same semantic base are presented: Unified Modeling Language (UML) and Agent Modeling Language (AML). The first graphic model is intended for general use and facilitates its users to analyze the ontology and dynamics of the modeled system. The second graphic model uses paradigms specific to agents and facilitates its users to create a design which is closer to the MAS which will be produced. After having discussed the relative merits of each of these graphic model types and presented some possible extensions, the chapter discusses the utility of, and a method for, documenting a multiagent model. In order to do this, the Overview, Design concepts, Details (ODD) protocol, which guides the modeler in the creation of a documentation of the objectives, constitutive elements and specific properties of the model, is presented.