Calculating x-ray tube spectra provides a comprehensive review of the modelling of x-ray tube emissions, with a focus on medical imaging and radiotherapy applications. It begins by covering the relevant background, before discussing modelling approaches, including both analytical formulations and Monte Carlo simulation. Historical context is provided, based on the past century of literature, as well as a summary of recent developments and insights. The book finishes with example applications for spectrum models, including beam quality prediction and the calculation of dosimetric and image-quality metrics.