The structural performance of steel fibre reinforced concrete (SFRC) composites, including ultra-high-performance concrete (UHPC), is sensitive to the orientation and distribution of fibres achieved during casting. Fibre distribution resulting from a concrete flow during casting is generally non-uniform, biased, and challenging to determine. This thesis proposes novel methods for non-invasive in situ characterisation of fibres dispersion in concrete. X-ray micro-computed tomography(micro-CT) is established as a benchmark nondestructive test (NDT) for measuring fibre dispersion, including in industrial applications. In this study, a new image analysis method is developed using a combination of local gradient and morphological operations for the time-efficient extraction of fibre information from 3D micro-CT images. The method balances analysis speed with accurate segmentation of individual fibres, including those that are in contact. The method provides greater than 90% accuracy of identifying individual fibres, with negligible errors in fibre volume, locations, and orientation estimations. Analysis of cores extracted from a UHPC bridge girder and tensile test specimens for a shotcrete tunnel lining showed high fibre orientation bias, highlighting the need for quality assessment in SFRC products. In this thesis, new stereological formulations are developed to obtain 3D fibre dispersion estimates from lower-dimensional measurements. For instance, estimating fibre volume (3D) from fibre area measurements (2D) using stereology makes NDT methods like X-ray projected images more feasible for in-situ characterisation. A good estimation of fibre volume fraction and spatial distribution is obtained from low-contrast, noisy X-ray projections of SFRC. Estimating the 3D fibre orientation state as a tensor from NDT data is formulated as an inverse problem solvable by standard optimisation techniques, with the approach applicable to most NDT methods. Experimental validation of the method using synthetic tomograms and UHPC cubes yielded accurate fibre orientation estimates. Also studied is the feasibility of replacing high-energy X-ray radiation with lower-energy microwaves for in situ imaging of SFRC, with supporting electromagnetic simulations. The results show that fibre volume and orientation tensor are derivable from the scattering response of steel fibres. The area is identified as one that is promising for future research.