As complex models are used in practice, modelers require ways of abstracting their models and having the ability to traverse levels of abstraction. The use of abstraction in modeling is spread over many disciplines and it is often difficult to locate an abstraction methodology or a set of practical techniques to help the modeler to perform the abstraction. Several approaches have been discussed in the general simulation literature: (1) variable resolution modeling; (2) combined modeling; (3) multimodeling; and (4) metamodeling. Our premise is that there are two different approaches to abstraction: behavioral and structural. We present one physical example of heat transfer and display the different abstraction approaches on this example. The approach taken to abstraction is an important design approach---to break a system into hierarchical levels. Behavioral abstraction serves to simplify the dynamic of a system without gaining the kind of reductionist knowledge one obtains through hierarchical decomposition. This work provides a comprehensive approach to system abstraction, while including specific practical behavioral methods to achieve abstract system descriptions.