The modal temporal logic based on time intervals that Halpern and Shoham introduce is a generalization of point-based temporal logic. The need for assertions to refer to time intervals rather than time points emerged from artificial intelligence areas like qualitative physics and automatic planning, but this work is intended to be a general vehicle for representing temporal information in what would be considered in many situations a more natural way. Intervals are not defined as primitive objects, however; the authors extend the point-based modal temporal logic by simply replacing the notion of satisfaction by a state with the notion of satisfaction by an ordered pair of states.

Syntax and semantics are defined both informally and formally. Halpern and Shoham use six modal operators to build well-formed formulas. The logic is general enough not to assume any connection between the truth value of a proposition over an interval and its truth value over any part of that interval.

Even more interesting, the only assumption made with respect to the underlying temporal structure is that the set of time points that lie between two points is totally ordered. Other time constraints--discreteness, linearity, density, and completeness--are not imposed. Halpern and Shoham characterize the expressiveness of their logic by capturing the definition of those constraints in the logic itself. The issue of time structure proves to be extremely sensitive for the validity problem for this logic. The authors dedicate a full section of their paper to studying this problem for different time constraints, and they prove that for most interesting classes of temporal structures, validity and satisfiability are undecidable.

Halpern and Shoham relate their work to other work in the field, especially process logic. Some previous knowledge of modal logics might be useful for fully understanding this valuable paper.