Development of mathematical models for internal combustion engines has received the attention of several researchers in the last two decades. The goal is to generate mathematical models which can be used to design future engines and improve the present ones. However, a mathematical model can be used as an engine design tool only if it has been proved to be reliable by way of conducting an extensive validation exercise. A general philosophy has been outlined which will help one select the system configurations to generate experimental data for a validation exercise. The related questions on interpretation of measured data have also been examined. Variation of model constants to generate model results for different system configurations has also been considered. The above ideas have been put forth in the form of ten important statements along with their explanations and illustrations given in the context of I.C. engines. The statements as such are relevant to any system in general and are not specific to I.C. engines only.