Trust may have many different informal definitions. In this work, formal definitions are proposed in Modal Logic in order to have clear rules for reasoning about trust. We start from trust in some properties of an information source, like sincerity, competence or vigilance, about a given proposition. Then, this definition is extended to trust about all the propositions which are about a given topic. A further extension is about all the propositions which inform about a given individual. Specific logics are (...) presented for reasoning about the fact that a given proposition is about a given topic and for reasoning about the fact that a proposition informs about a given individual. At the end, we give a brief extension to qualitative graded trust. (shrink)