We present here some geometric and structural results concerning the observer-based Failure Detection and Isolation problem in linear time-invariant continuous systems (in the deterministic case). The results are based on the dualization of some well-known results corresponding to both the disturbance rejection problem and the input-output decoupling problem. The results are presented via both the Geometric approach and the Structural approach. Both approaches rely on the structural properties of the monitored system. Since we just consider (strictly) proper systems, only the so-called finite zero structure is involved in the necessary and sufficient solvability condition for the considered failure detection and isolation problem. We only consider actuator failures.