Heat-setting is an important operation in industry for a variety of processes and products. Surprisingly, the scientific understanding of this subject. is very poor. . Recently a high-speed flexible thermomechnical analyser ('FTMA') has been built. This enables measurements. to, be made of the interrelation between tension, torque, length, twist, temperature and time for polymeric fibres and yarns under conditions relevant to commercial processing operations. Particularly novel features are the high rates of heating. and cooling obtainable - more than 1030K/s. The sensitivity of torque and tension measurements are 10-6 N. m and 10-5 N respectively. Experiments under-conditions somewhat analogous to those experienced by the filaments of a yarn in the heat-setting process, were carried out using the new thermo-mechanical analyser. In shrinkage tension constant geometry experiments, the monofilaments (nylon 6, nylon 6.6 and PET) were monitored at different setting temperatures over different time scales. The effects of different previous thermal histories and different heating rates on isothermal and dynamic shrinkage tension were studied. The results have been discussed in terms of unlocking of frozen-in distribution of strains on heating and temperature dependence of modulus with effect of thermal expansion coefficient on cooling. Stress build up after cooling has been evaluated as a major instrumental artefact. In torque-twist constant strain setting experiments, the behaviour of twist-set filaments (nylon 6 and PET) reveals that the setting-efficiency is dependent on temperature, time, twist level, previous thermal history and also on heating and cooling rates. However, markedly different setting behaviour for different directions of twist shows the presence of twist asymmetry in synthetic fibres. This suggests the possibility of the formation of a vortex in the spinnerette. In heat-setting the fibres, it has been experimentally demonstrated (during torsionäldeformation) that at the same average temperature, the addition of a sinusoidal fluctuation (approximately) can produce two opposite changes depending upon the frequency and amplitude of the temperature fluctuation. Finally, the reasonings leading to a scientific understanding of the mechanisms of heat-setting considering probable physical and structural parameters have been elucidated.