The knowledge required for action is generally indexical rather than o
bjective. For example, a robot that knows the relative position of an
object is generally able to go and pick it up; he need not know its ab
solute position. Agents may have very incomplete knowledge of their si
tuation in terms of what objective facts hold and still be able to ach
ieve their goals, This paper presents a formal theory of knowledge and
action, embodied in a modal logic, that handles the distinction betwe
en indexical and objective knowledge and allows a proper specification
of the knowledge prerequisites and effects of action, Several kinds o
f robotics situations involving indexical knowledge are formalized wit
hin the framework; these examples show how actions can be specified so
as to avoid making excessive requirements upon the knowledge of agent
s.