The invention relates to rotary units and rotary mechanisms that are suitable for use in numerous applications. Rotary units typically include rotational components that are configured to rotate. In some embodiments, for example, multiple rotary units are assembled in rotary mechanisms such that neighboring pairs of rotational components counter-rotate or contra-rotate relative to one another during operation of the rotary mechanisms. Rotational components generally include one or more implements that are structured to perform or effect one or more types of work as the rotational components rotate relative to one another in a given rotary mechanism. In certain embodiments, implements are configured to rotate and/or to effect the movement of other components as rotational components rotate. In some embodiments, engines include rotary mechanisms and are used in, for example, ground vehicles, marine vehicles, aircraft, or devices.