We propose a representation of concurrent actions; rather than invent a new formalism, we model them within the standard situation calculus by introducing the notions of global actions and primitive actions, whose relationship is analogous to that between situations and fluents. The result is a framework in which situations and actions play quite symmetric roles. The rich structure of actions gives rise to a new problem, which, due to this symmetry between actions and situations, is analogous to the traditional frame problem. In [Lin and Shoham 1991] we provided a solution to the frame problem based on a formal adequacy criterion called "epistemological completeness." Here we show how to solve the new problem based on the same adequacy criterion.