This paper proposes an extension of the event calculus which allows 
the representation of alternative situations (states-of-affairs) and 
beliefs. The basic idea of this <I>situated event calculus</I> is to use 
the notion of situations as developed in Situation Semantics, a 
linguistically motivated semantic theory. Instead of having events of the 
form 'Happens(Action,Time)' all events occur relative to a specific 
situation: 'Happens(Action,Time,Sit)'. In other words, an event can happen 
in one situation and not happen in an otherwise similar situation.
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Situations can thus be viewed as collections of events and are used here to 
represent beliefs: a person adopting a situation has knowledge of all the 
events occurring in it and believes all the fluents holding in it (i.e. all 
consequences of the events). Changing beliefs are modeled by adopting 
different situations. By using a circumscribed version of the event 
calculus and modeling 'adopt' and 'believes' as ordinary events and 
fluents, the frame problem is easily overcome, both wrt beliefs and 
"physical" fluents.