The interaction between ultrasound and the material structure of titanium alloys is critical to the nondestructive evaluation of many aerospace components. In this thesis, methods for characterising ultrasonic response and calibrating the results are reviewed, and appropriate measurements are reported from Ti-6A1l-4V and commercially pure titanium blocks. These same blocks are then sectioned, and the revealed metallography quantified. The observed structures are best described on three levels - individual alpha phase crystallites, the alpha plus beta grain structure, and larger 'macrograins'. The origins of these structures are discussed. Correlations are then sought between ultrasonic response and metallography, through direct mathematical modelling, graphical representations, and computer-based numerical methods. None of the available models are able to predict ultrasonic response accurately, although insight is gained to the behaviour of ultrasound and the causes of 'grain noise' - better termed microstructural backscatter. The concept of a backscatter coefficient is developed as a measure of the inherent 'noisiness' of a material when subjected to ultrasound, calculated for the range of blocks under test, and qualitatively correlated with microstructural features within the test blocks. The understanding gained is then fed back into an appreciation of theoretical models, the detectability of small flaws using ultrasound, and recommendations for future development of ultrasonic inspection.