Sliding mode control is an effective methodology for controlling systems with variable structures, providing a systematic approach to the problem of maintaining stability and consistent performance in the presence of modeling imprecision, uncertainties, and disturbances. The observation problem of the uncertain and nonlinear nuclear systems is investigated by using sliding mode observers (SMOs). Three different applications with uncertainties are under consideration: point reactor kinetics equations, nonlinear xenon dynamics, and a U-tube steam generator system. Numerical results are presented to illustrate the effectiveness of the SMO.