It deals with the problem of robust observer design. Traditional, observers are very sensitive to measurement noise and parameter uncertainties. In many cases, especially the so-called high gain observers tend to amplify noise. The original contribution is that an integrator is incorporated in the observer making it more robust to noise and giving it the property of attenuating measurement noise. The formulation given in this work is completely different to existing work in the literature. Several research teams have extended this work to multivariable and discrete cases. A collaborative work (Department of Paper Science, UMIST, and Manchester).