This paper explores the nature and structure of scientific models by taking the position that representations are not models themselves. Representations of models are essential tools for communicating and conversing about the scientific models underlying them. A scientific model is considered to be a set of ideas that describe a natural process. A scientific model conceived in this way can be mentally run, given certain constraints, to explain or predict natural phenomena. It is in this way that scientific models are both desirable products of scientific research and useful as guides to future research. A discussion of some examples of scientific models and how they are used in scientific inquiry and instruction is provided. (DDR)