 It must also be mentioned here that inspired by several sources among which the work on knowledge and belief updates for individual agents as described by DEL and DDL, combined with work on knowledge in groups of agents such as common knowledge (see, e.g., Meyer and Van der Hoek 1995), a whole new subfield has arisen, which can be seen as the multi-agent (counter)part of Dynamic Epistemic Logic. This deals with matters such as the logic of public announcement, and more generally actions that have effect on the knowledge of groups of agents. This has generated quite some work by different authors such as Plaza (1989), Baltag (1999), Gerbrandy (1998), Van Ditmarsch (2000), and Kooi (2003). For example, public announcement logic (Plaza 1989) contains an operator of the form [\(\phi]\psi\), where both \(\phi\) and \(\psi\) are formulas of the logic, expressing “after announcement of \(\phi\), it holds that \(\psi\)”. This logic can be seen as a form of dynamic logic again, where the semantic clause for [\(\phi]\psi\) reads (in informal terms): [\(\phi]\psi\) is true in a model-state pair iff the truth of \(\phi\) in that model-state pair implies the truth of \(\psi\) in a model-state pair, where the state is the same, but the model is transformed to capture the information contained in \(\phi\). Also in the other approaches the transformation of models induced by communicated information plays an important role, notably in the approach by Baltag et al. on action models (Baltag 1999, Baltag and Moss 2004). A typical element in this approach is that in action model logic one has both epistemic and action models and that the update of an epistemic model by an epistemic action (an action that affects the epistemic state of a group of agents) is represented by a (restricted) modal product of that epistemic model and an action model associated with that action. (See Van Ditmarsch et al. 2007, p. 151; this book is a recent comprehensive reference to the field.)