This paper considers the design of robust e 1 estimators based on multiplier theory (which is intimately related to mixed st.ructured singula,r value theory) and the applica-t~ioii of robust. tl est,imators to robust fault detection. The key to wtinintor-based, robust fault detection is to generat.e resitlnals which are robust aga.inst pla.nt uncertainties and cstenial dist,urbance inputs, which in turn requires the tlesigii of robust estimators. Specifically, the Popov-Tsppkin multiplier is used to develop an upper bound on an f 1 cost fiinct,ion over an uncertainty set. The robust tl estiniat.ion problem is formulated a.s a parameter optimization problem in which the upper bound is minimized siiljjwt to a Riccati equation constra,int. The estimation algoritliiii has t,wo stages. The first stage solves a mixed-~ioriu H 2 / f , estimation problem. In the second stage the f l estimator is made robust. The robust Cl estimation framcwork is then applied to robust fault detection of dyiianiic: systems.