Abstract Models provide a means to represent our understanding of the interaction among the components of a system. Individual instantiations of a system are captured by allowing the model to have parameters that may be different for each instantiation. Included in the model is a representation of the means to observe certain aspects of a system. Given the observations, successful estimation of the parameters requires that the parameters be identifiable. This is an important consideration for potential use of the model in diagnostic medicine or analysis and planning of experiments. Models considered here are typical of those used to describe flows and concentrations associated with ligand-receptor interactions.