In systems that require low energy consumption, voltage scaling is an invaluable circuit technique. It also offers energy awareness, trading off energy and performance. In wireless handheld devices, the communication portion of the system is a major power hog. We introduce a new technique, called modulation scaling, which exhibits benefits similar to those of voltage scaling. It allows us to trade off energy against transmission delay and as such introduces the notion of energy awareness in communications. Throughout our discussion, we emphasize the analogy with voltage scaling. As an example application, we present an energy aware wireless packet scheduling system.