  In this paper we introduce {\em global and local announcement logic} (GLAL),
a dynamic epistemic logic with two distinct announcement operators --
$[\phi]^+_A$ and $[\phi]^-_A$ indexed to a subset $A$ of the set $Ag$ of all
agents -- for global and local announcements respectively. The boundary case
$[\phi]^+_{Ag}$ corresponds to the public announcement of $\phi$, as known from
the literature. Unlike standard public announcements, which are {\em model
transformers}, the global and local announcements are {\em pointed model
transformers}. In particular, the update induced by the announcement may be
different in different states of the model. Therefore, the resulting
computations are trees of models, rather than the typical sequences. A
consequence of our semantics is that modally bisimilar states may be
distinguished in our logic. Then, we provide a stronger notion of bisimilarity
and we show that it preserves modal equivalence in GLAL. Additionally, we show
that GLAL is strictly more expressive than public announcement logic with
common knowledge. We prove a wide range of validities for GLAL involving the
interaction between dynamics and knowledge, and show that the satisfiability
problem for GLAL is decidable. We illustrate the formal machinery by means of
detailed epistemic scenarios.
