We propose a model of an agent which is both logical and operational. Our model of vivid agents takes into account that agents need not only the ability to draw inferences but also to update their current knowledge state, to represent and to perform (and to simulate the execution of) actions in order to generate and exe- cute plans, and to react and interact in response to perception and communication events. We illustrate our formalization of this basic functionality of an agent by means of examples. We also show how our model fits into the transition system semantics of concurrent reactive systems by identifying the five basic transitions of vivid agent systems: perception, reaction, planning, action, and replanning.