The expressive power of branching time logics is studied in the framework of the theory of ω-automata and ω-languages. The systems CTL* (computation tree logic) and ECTL* (extended computation tree logic) are characterized in terms of star-free, resp. regular ω-languages. A further characterization of CTL* by a "non-counting property" for sets of trees shows that it is decidable whether an ECTL*-formula can be written as a CTL*-formula.