  Free quantum field theories on curved backgrounds are discussed via three
explicit examples: the real scalar field, the Dirac field and the Proca field.
The first step consists of outlining the main properties of globally hyperbolic
spacetimes, that is the class of manifolds on which the classical dynamics of
all physically relevant free fields can be written in terms of a Cauchy
problem. The set of all smooth solutions of the latter encompasses the
dynamically allowed configurations which are used to identify via a suitable
pairing a collection of classical observables. As a last step we use such
collection to construct a $*$-algebra which encodes the information on the
dynamics and on the canonical commutation or anti-commutation relations
depending whether the underlying field is a Fermion or a Boson.
