'Earthquake Occurrence' provides the reader with a review of algorithms applicable for modelling seismicity, such as short-term earthquake clustering and pseudo-periodic long-term behaviour of major earthquakes. The concept of the likelihood ratio of a set of observations under different hypotheses is applied for comparison among various models. In short-term models, known by the term ETAS, the occurrence space and time rate density of earthquakes is modelled as the sum of two terms, one representing the independent or spontaneous events, and the other representing the activity triggered by previous earthquakes. Examples of the application of such algorithms in real cases are also reported. The book also introduces a number of computer codes developed by the authors over decades of seismological research.