For the determination of organ doses and the effective dose equivalent either physical or mathematical exposure models have to be used in order to establish suitable conversion factors between the dose values considered and certain measurable quantities. As the measurement of dose distributions in a realistic phantom of the human body is restricted to special situations, most of the conversion factors needed have to be derived by theoretical calculations using mathematical human phantoms and, usually, Monte Carlo methods. For these calculations, various more or less simplified models of the radiation source, the human body and the interactions between radiation and matter have been introduced. In this work, some examples of the influence of details in modelling on the resulting organ doses are demonstrated.