The active fault-tolerant control approach relies heavily on the occurred faults. Higher performances and more rigorous security requirements have invoked an ever increasing demand to develop real time fault detection and isolation system. The problem of fault diagnosis using analytical redundancy (model-based) methods has received increasing attention during recent years due to the rapid growth in available computer power. The main objective is to design and maintenance a fault-tolerant control system which guarantees a high overall system reliability and dependability both in nominal operation and in the presence of faults. Such an objective is achieved by a control performance index, which is proposed based on system reliability analysis. The methods involve generation and evaluation of signals that are accentuated by faults that have actually occurred. The procedures for generating such signals, called residuals, are based on two main distinct approaches. Direct approach consists in the elimination of all the unknown variable , keeping input-output relations involving only observable variables. Indirect approach estimates states, outputs or parameters in order to generate discrepancy signals obtained by the difference between the actual variables and their estimates.