Variability of surgical times heavily affects efficiency and utilisation of the operating room. This chapter develops a data-driven mathematical model that characterises the actions of the surgical process and their contribution to the total surgical time, including variability. The model gives insight into the surgical process, without the need to analyse a large number of surgeries for application of statistical methods. A surgical flow chart of cataract surgery is constructed by observing 85 cataract surgeries performed at Leiden University Medical Center (LUMC), combined with expert opinion. Markov chain analysis, based on this flow chart, is used to analyse the surgical process. The model identifies the sources of delay and variability in the surgical process and provides a structured way of analysing surgeries. The obtained surgical time distribution approximates the empirical surgical time distribution of cataract surgeries performed in 2009 and 2010 at LUMC. The model developed in this chapter may be used to study the influence of modifications in the surgical process and to predict the resulting surgical times. It can easily be adapted to analyse future surgical processes or to represent different surgical procedures.