 In the following sections, we will look at the many model-changing actions that have been studied in Dynamic Epistemic Logic. Many natural applications and questions arise as part of this study, and we will see some of the results obtained in this work. Along the way it will be convenient to consider many variations of the general formal setup described above. Despite these differences, at the core is the same basic idea: new modalities describing certain application-specific model-transforming operations are added to an existing logical language and the study proceeds from there. Proceeding now ourselves, we begin with what is perhaps the quintessential and most basic model-transforming operation: the public announcement. 