In probability theory, processes that show regularities if they are observed frequently are being looked at. These regularities consist in the relative frequency of special outcomes that become stabilised in long series of experiments. This leads to probabilities. In a second step, probabilities are defined more generally and detached from the performing of the experiments. For special random experiments the probabilities emerge in an obvious, simple way. Specific experimental questions lead to further terms of probability theory. Of special interest are the conditional probabilities and the independence of events.