To be able to predict the impact of any radiotherapy treatment the physics of radiation interactions and the expected biological effect for any radiotherapy treatment situation (dose, fractionation, modality) must be both understood and modelled. This review considers the current use and accuracy of the linear quadratic model which can be used to consider the variation in tissue response with fraction size. Cell kill following radiation damage results from damage to the DNA which can take a variety of forms. In many cases the linear quadratic model is used to estimate the relative impact for different situations especially clinical studies relating to fraction size. This is mainly undertaken using parameters derived from the linear quadratic model such as biological effective dose and standard effective dose. The model has also been adapted to consider the effect of overall treatment time, repair during treatment (as occurs for brachytherapy treatments) and other situations. There are some concerns over its use, mainly in the small dose ranges (both total low doses and low doses per fraction) where studies have shown its inaccuracy. In other situations however it does appear to provide a reasonable estimate of relative clinical effect. As with all models, however results should never be considered out of clinical context.