The nature of system identification in biological systems is analysed, in order to show what are the special features of biological systems in relation to the identification process, and to reveal what general lessons for measurement and control can be learned from the quantitative study of biological systems. Approaches to be adopted in the identification process are ou tlined, highlighting the importance of adopting an appropriate mathematical model. It is shown that biological systems constitute a paradigm for the application of identification procedures in the observational sciences generally. The paper sets the scene for the four application studies which follow, illustrating the application of identification methods to mechanical parameters of the lung, to elements of the neuromuscular system, to the blood pressure control system and in pharmacokinetics.