Computer simulations reproduce and predict the behavior of a system using a mathematical model, in particular if this model is too complex for analytical solutions. Computer simulations became an indispensible tool for the mathematical modeling of many natural systems in physics (computational physics), astrophysics, climatology, chemistry and biology, human systems in economics, psychology, social science, and engineering. In biology the most prominent and most challenging applications of computer simulations are to create a quantitative and predictive model of the cell (systems biology and mathematical biology), to predict the folding and binding of proteins (bioinformatics and theoretical chemistry) and to create a synthetic brain (Human Brain Project). In this intro lecture I will give an overview over the various areas in biology in which computer simulations are frequently used and the different methods that are commonly used to implement them.