ABSTRACT Currently, Ground-penetrating radar technology, that uses electromagnetic radiation in the microwave spectrum, has many applications in several fields and it is a non-destructive testing method that allows the rapid prospecting of building structures. In fact, in the last years Ground-penetrating radar technique has gained remarkable popularity for the assessment of physical conditions and diagnose of building structures and materials, such as concrete or stone. The general principle of prospecting with Ground-penetrating radar is based on the propagation of electromagnetic pulses. An antenna located on the material surface radiates energy downward into the material. This energy is partially reflected at interfaces where there is a dielectric contrast. The response of the material, consisting of all the reflected waves, is then recorded, processed and analyzed in order to characterize and/or study the material or medium by which they travel. This thesis deals with the ability of the Ground-penetrating radar, as a non-destructive testing technique, to prospect, diagnose and analyze the strength of structural timber. The non-destructive nature of this technique makes it particularly suitable for prospecting timber on site when it is part of a structure. The thesis is divided into four parts. The first part introduces all the necessary aspects for understanding the basis of Ground-penetrating radar technique. Likewise, the main physical properties of timber as well as the current standards for structural use are reviewed. After that, the tests undertaken to calibrate the performance and behaviour of the employed antenna (of 1.6 GHz central frequency) both in the air and on timber are described. In the second part, the main physical properties of wood are prospected by means of Ground-penetrating radar. In particular, in this part of the thesis, these properties which decisively affect the mechanical strength of timber are analysed. Firstly, the dielectric anisotropy of wood is studied and thereby the aptitude of Ground-penetrating radar to reveal the dielectric anisotropy due to the different grain directions. Secondly, tests are conducted to assess how the electromagnetic radiation is affected by the moisture content variation of timber, since the moisture content is one of key factors influencing the degradation or deterioration of a piece. Finally, the ability of Ground-penetrating radar to distinguish different species of wood from the density analysis is tested. In the study of each of these properties, given the novelty of this application, both methodological aspects and quality and reliability of the results are thoroughly discussed. In the third part of the thesis, the results, obtained from the Ground-penetrating radar study to evaluate the dielectric anisotropy, the moisture content and the density, are compared with those obtained with two emerging non-destructive techniques in the structural timber field of study (the resistograph and ultrasound technique). The results show that the Ground-penetrating radar is a more reliable technique for the thesis target. However, a combination of all of them might provide interesting data for evaluating the wood condition and for assessing a diagnosis. In the fourth and last part, the basic criteria for using Ground-penetrating radar technique as a strength grading tool are established. Currently the research agenda is focussed on developing non destructive grading methods of timber and on providing records on variables that might be associated with strength parameters. Regarding this, a thoroughly study comparing the different techniques available currently and the grading defined by means of Ground-penetrating radar is performed. The defined criteria to carry out the Ground-penetrating grading are established for the very first time in this thesis.