This paper describes a new representation of the sensing and control uncertainties that occur in sensor-based robot control. A sensor is represented by three quantities: a domain, which is the set of robot configurations at which a valid measurement can be taken; an absolute sensing uncertainty field, which describes the sensor's absolute (global) accuracy; and an incremental motion uncertainty, which describes the sensor's relative (pertaining to displacements) accuracy. Control uncertainty represents the ability of a controller to drive the measured error near zero, regardless of how accurate the measurement is. These descriptions of sensing and control capability determine the evolution of uncertainty in a sensor-based motion plan. Sensor fusion is handled in this representation as the simultaneous activation of multiple sensors. A composite sensor's representation is computed from the representation of its constituent sensors. A motion planning framework based on the above representation of uncertainties is introduced through an example. This framework is designed to utilize the available sensing modes required to accomplish a task.