X-rays can be used as a sensitive, non-destructive probe for the characterisation of semiconductors. The energy and wavelength of X-rays is such that structural information down to the Ảngstrom level can be yielded with a depth penetration of several microns. As a result X-rays are ideally suited to the study of thin layer semiconductors. Double crystal diffractometry in particular is widely used throughout industry for the characterisation of heteroepitaxial layers. As epitaxial growth techniques become more sophisticated the demand for more detailed structural information becomes even greater. In particular, the trend towards thinner and thinner layers in optoelectronic devices means that conventional characterisation methods are often lacking in sensitivity. This thesis concentrates on the development of new techniques used in the study of ultra thin epitaxial layers. Skew beam paths have been utilised to provide enhanced sensitivity to thin surface layers. By choosing a suitable asymmetric reflection and rotating the sample through the reflection plane normal it is possible to tune the angle of incidence to that required. Experiments performed on a single epilayer yielded a fourfold increase in intensity of the layer diffraction effects compared to a conventional grazing incidence asymmetric reflection. Two layer structures have been characterised using Pendellӧsung fringes. Although a conventional technique it has only recently been realised that errors may result in layer thickness determination due to direct measurement of fringe spacing from the rocking curve. Fourier Analysis has been used to accurately determine layer thickness and the conditions necessary for its use have been fully investigated. By evaluating layer thickness in this fashion the process of matching theoretical rocking curves with those produced in experiment becomes more straight forward and close fits have been achieved. X-ray reflectivity is a well established method sensitive to electron density change, although as yet it has had little application in the study of epitaxial layers. Angular reflectivity using both double and triple crystal diffractometers has been used to characterise thin epilayers, and the relative merits of each are discussed. A technique known as energy dispersive reflectivity has also been investigated and found to be an extremely rapid method of determining layer thickness. Theory describing X-ray reflectivity is discussed and a computer program has been written to model the experimental results.