A method of quantifying lung function of a subject comprising: a step of obtaining plural measurements of the amounts at the mouth of plural respiratory gases in inhaled and exhaled breath as a function of time within each breath of a plurality of successive breaths during a period of steady breathing and a period of inert gas wash-in or wash-out of a respiring subject; a step of outputting the measured amounts to a data processor adapted to model lung function by using a parameterised lung model adapted to predict expired amounts of the plural respiratory gases at the mouth of a respiring subject; a step of varying the parameters of the lung model to fit the predicted expired respiratory gas amounts to the plural measurements; and a step of outputting at least one parameter of the lung model as a quantifier of lung function characterized in that: the parameterised lung model is adapted to predict expired amounts of the plural respiratory gases at the mouth of a respiring subject at the time point of each of said plural measurements, the parameterized lung model models the lung as a plurality of alveolar compartments (VA1, VA2, VAi) connected by a respective plurality of personal deadspaces (VD1, VD2, VDi) to a common deadspace leading to the mouth, and wherein the volumes of the personal deadspaces (VD1, VD2, VDi) are distributed with alveolar compartment volume (VA1, VA2, VAi) according to a personal deadspace distribution; and the step of varying the parameters of the lung model to fit the predicted expired respiratory gas amounts to the plural measurements comprises fitting the predicted expired respiratory gas amounts at each time point to the expiratory flow profile over each breath of the multiple successive breaths for one or more of the respiratory gases. a step of obtaining plural measurements of the amounts at the mouth of plural respiratory gases in inhaled and exhaled breath as a function of time within each breath of a plurality of successive breaths during a period of steady breathing and a period of inert gas wash-in or wash-out of a respiring subject; a step of outputting the measured amounts to a data processor adapted to model lung function by using a parameterised lung model adapted to predict expired amounts of the plural respiratory gases at the mouth of a respiring subject; a step of varying the parameters of the lung model to fit the predicted expired respiratory gas amounts to the plural measurements; and a step of outputting at least one parameter of the lung model as a quantifier of lung function characterized in that: the parameterised lung model is adapted to predict expired amounts of the plural respiratory gases at the mouth of a respiring subject at the time point of each of said plural measurements, the parameterized lung model models the lung as a plurality of alveolar compartments (VA1, VA2, VAi) connected by a respective plurality of personal deadspaces (VD1, VD2, VDi) to a common deadspace leading to the mouth, and wherein the volumes of the personal deadspaces (VD1, VD2, VDi) are distributed with alveolar compartment volume (VA1, VA2, VAi) according to a personal deadspace distribution; and the step of varying the parameters of the lung model to fit the predicted expired respiratory gas amounts to the plural measurements comprises fitting the predicted expired respiratory gas amounts at each time point to the expiratory flow profile over each breath of the multiple successive breaths for one or more of the respiratory gases. the parameterised lung model is adapted to predict expired amounts of the plural respiratory gases at the mouth of a respiring subject at the time point of each of said plural measurements, the parameterized lung model models the lung as a plurality of alveolar compartments (VA1, VA2, VAi) connected by a respective plurality of personal deadspaces (VD1, VD2, VDi) to a common deadspace leading to the mouth, and wherein the volumes of the personal deadspaces (VD1, VD2, VDi) are distributed with alveolar compartment volume (VA1, VA2, VAi) according to a personal deadspace distribution; and the step of varying the parameters of the lung model to fit the predicted expired respiratory gas amounts to the plural measurements comprises fitting the predicted expired respiratory gas amounts at each time point to the expiratory flow profile over each breath of the multiple successive breaths for one or more of the respiratory gases.