Validation of simple and complex models in physiology and medicine

The problem of assessing the value of mathematical models in physiology and medicine is considered. The role of validation within the modeling process is clearly defined. An appropriate vocabulary and validation procedures to be adopted are outlined for simple and complex models, with these two classes defined operationally on the basis of theoretical identifiability. Some examples illustrating these validation procedures are briefly discussed. It is shown that simple and complex models each have a role both in physiology and clinical application when properly validated.