The work presented in this thesis was carried as part of a project to improve the analysis of clinical data obtained by microwave thermography. For microwave thermography measurements to be usefully interpreted for detecting thermal anomalies in the human body at depths of up to several centimetres, the thermal and microwave dielectric properties of tissues must be known. This thesis is mainly concerned with the measurement and interpretation of the values of the thermal conductivity and diffusivity of human and animal tissues. The thermal properties of biological tissue are required, in conjunction with a bio-heat equation, to allow the formation of computational models to simulate the temperature distribution inside the human body. These computational models are also useful in the analysis of tomographic temperature measurements, and are essential to ensure accurate heating in hyperthermia. The Pennes conventional bio-heat equation has proven to be successful in analysing the data produced by microwave thermography. The thermal properties of biological soft tissue are dependant on the tissue water content. Water is a major constituent of most soft biological tissues, and it has a higher thermal conductivity and thermal diffusivity than any other constituent of biological tissue. The thermal properties of biological tissue can be modelled using a mixture equation, which describes the behaviour of a two phase system in terms of the thermal properties of the individual constituents and their relative volume fractions. This allows the variation of the thermal properties of biological tissue with water content to be analysed. A self-heating thermistor probe system was used in this study to measure the in-vitro thermal conductivity and thermal diffusivity of a wide variety of human and animal tissues. The system was calibrated using glycerol and agar-gelled water since the thermal behaviour of these materials and mixtures of these materials was well known. The calibration data was examined to determine the accuracy of the calibration and to determine if there was a relationship between the observed thermal conductivity and thermal diffusivity which was generated by the measurement system