The measurement of moisture content within construction materials is a useful capability for researchers investigating the building physics associated with material durability, and also for building professionals seeking to diagnose the cause of building defects. Various tools have been employed by both groups. However, research tools are often impractical for use on site, and highly portable techniques used by building professionals often have limited accuracy and applicability. This study investigates the use of Time Domain Reflectometry (TDR) for in-situ measurement of moisture content within building materials; this technique has the potential to be useful for both user groups. Time Domain Reflectometry has established applications in the soil sciences. The basis for the technique is measurement of the relative permittivity of a dielectric medium adjacent to waveguides; this is dominated by the presence of moisture. This study has developed probes suitable for installation by carefully drilling holes within construction materials. Experimental work has been undertaken to evaluate the TDR response to changing moisture content in a range of materials. The study has demonstrated that the technique will assess long term trends in relative moisture content of a porous material with minimal preparation. Measurement of absolute moisture content is found to be considerably more complicated, being particularly influenced by the contact quality between the probe and the measured material. Experimental work has explored the contact quality associated with the TDR measurement; this has resulted in a mathematical description proposed to account for the contact quality with the TDR measurements. Statistical reanalysis of experimental results using the contact quality model has successfully demonstrated an improved evaluation of absolute moisture content consistent with other direct laboratory measurement techniques. Finally, the study identifies considerations for others interested in using TDR for measuring moisture content within a solid material. The study also suggests potential areas of research which would enhance the capability of TDR to measure moisture content within construction materials.