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Experimental observations linked to a theoretical analysis of the so-called "self-mixing interference" in semiconductor lasers are presented, and several schemes using the self-mixing technique applied to the measurement of various physical parameters are proposed in this thesis. A theory of self-mixing interference inside a single-longitudinal-mode diode laser is developed, based on steady-state equations of the lasing condition in a Fabry-Perot type laser cavity. The resulting theoretical models are first presented, and through them the necessary theory for an analysis of the self-mixing interference in a single-mode diode laser is given. It was shown that the optical intensity modulation produced by an external optical feedback was due to the variations in the threshold gain and the laser spectral distribution of the device used. The gain variation results in an optical intensity modulation, and the spectral variation determines both the modulation waveform shapes and the coherence properties of the interference. The theoretical analysis of the self-mixing interference yielded a simulation of the laser power modulation which was then investigated experimentally. The semiconductor laser used in the experiment functions not only as a conventional light source but also as a self-aligned interferometer and a detector. The laser sends light, either in free space or through an optical fibre, to a changing target from which the optical backscatter is fed back into the laser and detected by the internal photodetector. This self-mixing effect inside the laser cavity results in the laser power variation which is related to the changes of the external physical parameters. The monitoring of the laser power thus provides a simple method for optical sensing. In the experiments performed, three significant conclusions are drawn: (i) the occurrence of the self-mixing interference is not dependent on the initial coherence length of the diode laser in the absence of external optical feedback, (ii) the interference is not dependent on the use of a single-mode or a multi-mode laser as the source and (iii) the interference is independent of the type of fibre employed, i.e. whether it is single-mode or multi-mode. Comparison of this kind of interference with that in a conventional interferometer shows that (i) self-mixing interference has the same phase sensitivity as that of the conventional arrangement; (ii) the modulation depth of the interference is comparable to that of a conventional interferometer and (iii) the directional information of the phase movement and that of a moving object scattering the light can be obtained from the sawtooth-like interference signal. The above factors have significant implications for optical sensing of a wide range of physical parameters. Finally, several application schemes of the self-mixing interference technique are investigated, and the preliminary experimental results achieved highlight its significant advantages of simplicity, compactness and robustness, and the self-aligning, self-detecting abilities of self-mixing interferometry, when compared to the use of conventional interference methods.