New low-power, 8-bit, 16-bit, and 32-bit microcontrollers (MCUs) designed for portable and battery-powered handheld products include an impressive set of new features, including low power capabilities. Choosing the best MCU and taking advantage of design features based on the need of specific applications requires an understanding of the different low power modes that are available. This white paper explains features and functions with which designers must be familiar in order to optimize performance and make full use of these MCUs: flexible clocking systems, resource event-driven features, and — perhaps most importantly — the way to transition to and from standby and other low power modes.