  Sheaves of structures are useful to give constructions in universal algebra
and model theory. We can describe their logical behavior in terms of
Heyting-valued structures. In this paper, we first provide a systematic
treatment of sheaves of structures and Heyting-valued structures from the
viewpoint of categorical logic. We then prove a form of {\L}o\'s's theorem for
Heyting-valued structures. We also give a characterization of Heyting-valued
structures for which {\L}o\'s's theorem holds with respect to any maximal
filter.
