  The framework of locally covariant quantum field theory is discussed,
motivated in part using "ignorance principles". It is shown how theories can be
represented by suitable functors, so that physical equivalence of theories may
be expressed via natural isomorphisms between the corresponding functors. The
inhomogeneous scalar field is used to illustrate the ideas. It is argued that
there are two reasonable definitions of the local physical content associated
with a locally covariant theory; when these coincide, the theory is said to be
dynamically local. The status of the dynamical locality condition is reviewed,
as are its applications in relation to (a) the foundational question of what it
means for a theory to represent the same physics in different spacetimes, and
(b) a no-go result on the existence of natural states.
