This paper presents the design and implementation of balance control for a unicycle robot. The robot consists of a wheel, a body frame, and a reaction wheel. The wheel enables the robot to move forward or backward to obtain longitudinal stability, while the reaction wheel is used for obtaining lateral stability. Control of this system is a challenging task because of inherent nonlinearity, instability, and underactuation. A detailed dynamic model of the system is derived for control design. By retaining the predominant nonlinear terms and neglecting the high-order coupling terms, the system model is simplified to two decoupled systems. Sliding mode control and feedback linearization are then used to design the stabilizing controllers for the simplified models. The robot is constructed and the designed control schemes are realized through a digital signal processor. The effectiveness of the control schemes is demonstrated through experimental studies.