One of the important elements of a fault diagnosis and accommodation scheme is monitoring of plant states. Observers (state estimators) are commonly used for monitoring in fault diagnosis. In the first part of the paper the celebrated Doyle-Stein Observer (DSO) is studied as a monitoring tool with some features of robustness (insensitivity to parameter changes) in both the s domain and the time domain. The time domain representation of the DSO is derived. In the second part of the paper the Robust Kalman Filter (RKF) is examined and the relationship between the DSO and the RKF is given. The simulation study shows the insensitivity of the DSO to some parameter changes. Copyright (C) 1998 IFAC.