This thesis is concerned with an approach to the assessment of respiratory gas transport in individual subjects, which is based on the techniques of mathematical modelling. The general mathematical modelling approach to a physiological system while similar to that for a physical system is sufficiently different to warrant discussion (Chapter 1). Models have been frequently employed in the study of respiratory gas transport and the different models are reviewed in Chapter 2. A method of characterising these models is suggested. Many of the models consist of simple algebraic equations which describe steady-state conditions. An extension to these models to quantify ventilation-perfusion distribution is presented (Chapter 2 and Appendix 10). The main deficiencies of steady-state models are the restrictions which they impose on experimental conditions both limiting the information content of the experiment and making it difficult to perform tests on certain subjects. A new approach to the measurement of respiratory gas exchange is suggested based on dynamic as opposed to steady-state models and using the techniques of parameter estimation. The necessary experimental and computing techniques have been developed and details are presented in Chapter 3. The feasibility of this approach is proved by application to the study of inert gas wash-out experiments (Chapter 4). While this method of analysis can utilise the within-breath detail of the expired concentration measurements, the physiological mechanisms underlying this aspect of function are not fully clarified. An investigation of one of the relevant mechanism (Taylor diffusion) using a distributed model is also presented in Chapter 4. The techniques of dynamic modelling are applied to the development of a new non-invasive method for the measurement of cardiac output and CO2 lung volume. (Chapter 5). Models can also be of value for educational purposes and a special simulator of gas transport is presented in Appendix 4