Default Logic is one of the most prominent approaches to nonmonotonic reasoning, and allows one to make plausible conjectures when faced with incomplete information about the problem at hand. Default rules prevail in many application domains such as medical diagnosis and legal reasoning. Several variants have been developed over the past years, either to overcome some perceived deficiencies of the original presentation, or to realize somewhat different intuitions. This paper provides a tutorial-style introduction to some important approaches of Default Logic. The presentation is based on operational models for these approaches, thus making them more easily accessible to a broader audience, and more easily usable in practical applications.