In this study the use of a He-Ne laser as excitation source in fluorescent endoscopy is investigated Fluorescence endoscopy is based on the principle of injecting a fluorescent tumour localiser prior to examination with a fibre-optic endoscope. Tumours marked by the fluorescent localiser can then be identified by the detection of the characteristic fluorescence output of the localiser. The localiser examined in this research is Haematoporphyrin or Hp. Previous studies with Hp have generally utilised the violet portion of the visible spectrum (400nm) to excite fluorescence Spectroscopic studies carried out in this project reveal that He-Ne laser light (632 8nm) can be used to excite Hp fluorescence. It is shown that 632nm excitation possesses significant advantages over the use of violet light for the detection of small tumours deeply embedded in healthy tissue. One such type of tumour is early small cell carcinoma of the tracheo-bronchial (TB) tract. These tumours may only be a few millimetres in extent and up to 0 5mm thick Such a small tumour will have very small amounts of the Hp marker material. This combined with the fact that Hp has a very low excitation efficiency at 630nm will result in a very low fluorescent emission. To detect such a low level light signal a highly efficient illumination and detection system is required Such a system has been developed and is described in chapter 3. In vitro experimentation demonstrates that the system can detect fluorescence from concentrations of Hp down to as low as 0 01 pg/ml in methanol and 0 1 /¿g/ml in water. A comprehensive mathematical model has been developed to predict the response of the detection system for the detection of early small cell^TB tumours. The model predicts that the instrument can detect tumours as low as 2 5mm in extent and 0 3mm thick embedded deep within the submucosa of the TB tract. In order to measure the response of the instrument in conditions which simulate a clinical environment a tumour-tissue phantom has been developed. Experiments carried out on the phantom with the endoscopic apparatus yield results in very good agreement with those predicted by the model. In conclusion it has been shown that He-Ne laser light can be used as an effective excitation source in fluorescent endoscopy for the detection of previously undetectable tumours. The use of He-Ne laser excitation in fluorescent endoscopy could then play a role in the cancer screening of high risk sections of the population.